Interpenetrating polymer networks of poly(methacrylic acid) and polyacrylamide: synthesis, characterization and potential application for sustained drug delivery

Abstract

Novel interpenetrating polymer networks (IPN) of poly(methacrylic acid) (PMAA) and polyacrylamide (PAAm) were synthesized and characterized in terms of their swelling ability, microhardness and morphology. The potential of these new polymeric materials as a sustained delivery system for cationic drug was revealed. The study demonstrates that the IPN's composition is a powerful tool to control the IPN's structure and properties and hence their performance as a new polymeric system for sustained drug delivery.

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Introduction

Polymer hydrogels have great potential as materials for biomedical and pharmaceutical applications due to the unique combination of their properties and behavior.¹ They are porous materials when swollen and their mesh size could be easily tuned via the crosslinking density of the component networks – two peculiarities which permit the easy loading of drugs as well as the subsequent controlled drug release. Moreover, due to their high water content and similarity in physico-mechanical performance to body tissues, they are inherently biocompatible. Hydrogels could also possess smart behavior as some of them could respond in a defined manner to external stimuli as pH, temperature, electric field, magnetic field, etc.

Along with the above listed advantages, hydrogels suffer from several drawbacks regarding their drug delivery applications, namely: (i) they are brittle, which is a problem when used as drug eluting implants; (ii) they are not the best delivery system for hydrophobic drugs because the amount of the loaded drug and its homogeneous distribution are limited; (iii) the high water content and large pore sizes of hydrogels often result in rapid drug release.

These drawbacks could be overcome by utilizing the concept of interpenetrating polymer networks (IPN).² This is one of the approaches usually taken to improve polymer hydrogels' mechanical strength and stability. IPN possess entangled structure, which could be easily tuned, preventing in this way the fast drug release and exerting control over it. IPN could be produced either as bulk materials or as micro and nanogels depending on the envisaged application. Thus, they provide a versatile platform for drug delivery applications.

Recently we have demonstrated the potential of IPN based on poly(acrylic acid) (PAA) and polyacrylamide (PAAm) as sustained drug delivery system for a cationic drug (verapamil hydrochloride, VPM).³ We have outlined two main factors governing the drug delivery behavior of this system: (i) the IPN's functionality as well as (ii) IPN's network density. The acidic functionality of the PAA component was expected to control the interaction with the cationic drug (VPM), while the IPN's specific structure (phase separation at nano level) influenced the diffusion of the drug within the polymer network. Both factors were controlled simply through varying the IPN's composition.

Poly(methacrylic acid) (PMAA) has very similar structure however different properties as compared to PAA. The extra methyl group in PMAA results into a pH induced conformational transition in aqueous solutions at relatively low degrees of ionization.⁴ At low pH, PMAA chains are shrunk into compact coils and highly compact clusters from the PMAA chains, connected between themselves⁵ are formed. These hydrophobic clusters are formed via strong attractive hydrophobic forces between the PMAA backbone methyl groups (i.e. short-range interactions), which take place at low pH in addition to the hydrogen bonding between COOH groups.⁶ Upon a pH increase, the PMAA chains transform into expanded random coils thus resembling the well-known globule-to-coil transition in proteins. When pH increases above the pKa of PMAA (∼5.5), the repulsions between carboxylate anions start to dominate and the hydrophobic attractions are completely broken down, i.e. the chains expand to a water-swollen structure⁹ in an abrupt mode. PAA and PMAA also differ in their acid strength, the pK_a of PMAA being higher by 1 pH unit from the pK_a of PAA, i.e. PAA is 10 times stronger acid as compared to PMAA. Moreover, PMAA is known to be more biocompatible as compared to PAA.

All these peculiar properties of PMAA as compared to PAA provoked our interest towards developing IPN of PMAA and PAAm, a novel material which combines acidic functionality (imparted by PMAA) with strong propensity for hydrogen bonds formation imparted by both components, PMAA and PAAm. In this way the newly developed polymer material was designed to be an ideal candidate for the sustained drug delivery of a cationic drug. As model cationic drug we choose VPM. VPM is applied for treatment of angina pectoris and mild to moderate systemic hypertension.⁷ When used for antihypertensive therapy it must be taken several times daily. Thus due to its pharmacokinetics and physicochemical properties, it is a good candidate for developing controlled release formulations.

The aim of the present study was to synthesize and study novel IPN from PMAA and PAAm and to reveal the potential of these new materials as drug delivery system for a cationic drug, VPM. The effect of the substitution of PAA with PMAA in the IPN was evaluated in terms of IPN–VPM interaction as well as its influence on the IPN's sustained drug delivery performance.

Experimental part

Materials

Acrylamide (AAm, purum, ≥98.0%) was purchased from Fluka AG, Germany. Methacrylic acid (MAA, extra pure, 99.5%, stabilized) was purchased by Across Organics, Belgium. Potassium peroxodisulfate (PPS) and N,N′-methylenebisacrylamide (MBAA) were purchased from Sigma-Aldrich. Verapamil hydrochloride (VPM) was provided by Knoll AG, Germany. All reagents were used as received without further purification.

The chemical formula of both monomers as well as of the drug, VPM, are presented in Fig. S1.†

IPN's hydrogels synthesis

PMAA/PAAm IPN were synthesized via the sequential method. The 1^st single network (SN) from PMAA was prepared by a free radical crosslinking polymerization of MAA (1.16 M aqueous solution), 0.1 mol% PPS and 4 mol% MBAA (both % relatively to MAA) at 60 °C for 6 hours. Eight PMAA SNs were obtained, each of them was purified in distilled water to completely remove traces from non-reacted chemicals (the wastewaters were checked by UV). The conversion of MAA to PMAA was evaluated by applying a standard titration procedure. To this purpose, 5 ml aliquot from the collected wastewaters, used for purifying of one SN PMAA, was titrated by a standard aqueous solution of NaOH (0.105 N, determined by titration with HCl) and the quantity of the non-reacted MAA was calculated. The conversion of MAA to PMAA was determined to be 99% ± 1% for all synthesized SNs PMAA.

The 2^nd PAAm network was in situ synthesized into the 1^st SN PMAA. To this purpose seven dry SNs PMAA were transferred into five aqueous solutions with different AAm concentrations (0.5 M, 1 M, 2 M, 3 M, 4 M, 5 M, 6 M). The solutions contained also 0.1 mol% PPS and 0.1 mol% MBAA (both % relatively to AAm). The SNs PMAA were left to swell for 48 h until constant weight was reached. The in situ crosslinking polymerization of AAm in the SN PMAA took place at 60 °C for 6 hours. In this way seven IPN with various PMAA/PAAm ratios were obtained (Table 1). Each of the newly synthesized IPN PMAA/PAAm was washed with distilled water to completely remove traces from non-reacted chemicals (the wastewaters were checked by UV). The non-reacted AAm quantity was determined in order to obtain the exact IPN' composition. To this purpose the UV method was applied as described elsewhere.⁸ The AAm conversion to PAAm was determined to be 99% ± 1%.

Table 1 IPN PMAA/PAAm with different composition, φ^PMAA, and SNs PMAA and PAAm

Sample	PMAA	IPMA1	IPMA2	IPMA3	IPMA4	IPMA5	IPMA6	IPMA7	PAAm
φ^PMAA	1	0.86	0.71	0.52	0.39	0.30	0.26	0.20	0

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The weight fraction of PAA in the final IPN was determined using the equation:

φ^PMAA = m^PMAA/(m^PMAA + m^PAAm) (1)

where m^PMAA and m^PAAm are respectively the weights of the PMAA and PAAm in the IPN, while φ^PMAA is respectively the weight fraction of PMAA in the IPN.

For sake of comparison one SN PMAA and one SN PAAm were obtained. All SNs were used as referent samples (Table 1).

IPN's drug loading

Dry disk shaped pieces with diameter 10 mm from each IPN with a defined composition were immersed for 24 h in aqueous solutions of VPM with concentration 100 mg ml⁻¹. The swollen pieces were washed by distilled water and dried.

The VPM entrapment efficiency (EE) was calculated using the formula:

EE [%] = (m^o_VPM − m_{unloaded VPM}) × 100/m^o_VPM (2)

where m^o_VPM is the weight of VPM in the initial VPM solution from which the drug loading was carried out, while m_{unloaded VPM} is the weight of VPM remaining in the VPM solution after IPN loading.

Equilibrium swelling ratio (ESR)

20 disk shaped pieces with diameter 6 mm were cut from each IPN film at its equilibrium swollen state and each piece was weighted. Then, the pieces were dried until constant weight and their ESR was determined following the equation:

ESR = (m_swollen − m_dry)/m_dry (3)

here, m_swollen and m_dry are respectively the weights of the swollen and the dry piece. The final ESR value for each IPN was obtained after averaging the results for the 20 disk shaped pieces.

Vickers microhardness (MH)

The MH was measured by Leica VMHT apparatus, Leica Mikrosysteme GmbH, Wien, Austria at three different loads: 300, 500 and 1000 gf and indentation time 16 s. At each load, 5 indentations were performed and the results for the indentation diagonals were averaged. From the slope of the P vs. d² dependence, where P is the load in Pa and d is the indentation diagonal in mm, the Vicker's microhardness value (HV) in MPa was determined, using the equation:

HV = 1.854 × P/d² (4)

here, 1.854 is a constant reflecting the geometry of the pyramidal diamond indenter used for the Vicker's microhardness measurements.

Scanning electron microscopy (SEM)

The morphology of fractured surface of IPN with various composition (PMAA/PAAm ratio) was examined by scanning electron microscope (JSM-5510, JEOL, Japan) operating at 10 kV. To prepare the samples for imaging they were coated with gold for 30 s using a sputter-coater (JSC 1200, JEOL, Japan) in an inert argon atmosphere.

Temperature modulated differential scanning calorimetry (TM-DSC)

TM-DSC was performed on DSC apparatus Q200, TA instruments, USA. The temperature calibration was performed with sapphire disc supplied by TA instruments in Tzero aluminum pans (TA instruments) in the desired temperature interval. Samples with room humidity were tested in the same type of pans from 30 to 215 °C with 2 °C min⁻¹ heating rate, modulation amplitude 1 °C and a period of 60 seconds under nitrogen flow (50 ml min⁻¹).

In vitro VPM dissolution studies

Drug release profiles were evaluated using a dissolution test apparatus (Erweka DT 600, Germany). The USP paddle method was applied. The test was carried out at a paddle rotation speed of 50 rpm, in 500 ml dissolution medium maintained at 37 ± 0.5 °C with changing pH throughout the release experiments. To this purpose, the VPM loaded IPNs were first immersed in 0.1 mol l⁻¹ HCl solution (pH 1.2) for 2 h and then transferred into phosphate buffer solution (pH 6.8) up to 24 h. 5 ml aliquots of dissolution media were withdrawn at selected intervals up to 24 h. Each sample was filtered through a 0.45 mm membrane filter (Sartorius cellulose acetate filter, Germany). The quantity of the drug in the sample solution was determined by UV spectroscopy at 278 ± 2 nm using a Hewlett-Packard 8452 A Diode Array spectrophotometer, USA. The cumulative percentage of the drug release was calculated and the average of six determinations was used in the data analysis.

Results and discussion

Equilibrium swelling ratio (ESR) of PMAA/PAAm IPN

The ESR dependence on IPN's composition is shown in Fig. 1. Both neat SNs, PMAA and PAAm, have higher ESR as compared to the IPN samples. This difference could be explained by the higher network density of IPN as compared to SNs due to the inherent for IPN mutual penetration and interlacing of both networks (PMAA and PAAm). In addition, the hydrogen bonding between both components also contributes to the network density increase, respectively the ESR decrease. At neutral pH, the PMAA has comparatively low ionization degree and thus its chains have compact coil conformation which additionally enhances the impermeability of PMAA domains to water. In this way three factors have similar impact on the overall IPN's density, effectively increasing it, respectively, resulting into the observed ESR values decrease.

Fig. 1 Equilibrium swelling ratio dependence on IPN PMAA/PAAm composition.

When comparing the ESRs of the IPN only, a trend of an ESR decrease is clearly seen as the PMAA content increases, i.e. the φ^PMAA increase results into an ESR decrease. IPMA7, the IPN with the lowest PMAA content (φ^PMAA = 0.2), shows the highest ESR value. This observation is exactly the opposite when compared to the IPN of PAA and PAAm studied previously.³ There, the ESR increased with increasing the polyacid (PAA) content in IPN. The observed difference in the ESR dependence on the IPN composition when comparing PMAA/PAAm and PAA/PAAm IPN could be due to the following factors:

• PMAA is more hydrophobic and shows lower ability to swell in water as compared to the neat PAA especially at low and medium pH, i.e. at low dissociation degree (water is bad solvent for PMAA in non-dissociated state⁹). When comparing both acidic SNs, PMAA and PAA, obtained at the same monomer, initiator and crosslinking agent concentrations, temperatures, polymerization time, etc., the ESR of the neat PAA is ∼7,³ while for the neat PMAA it is ∼5 (Fig. 1).

• PMAA chains form highly compact hydrophobic clusters connected between themselves by short extended parts of the PMAA chains at low pH.⁵ The clusters are formed via strong attractive hydrophobic forces between the PMAA backbone methyl groups (i.e. short-range interactions) and they additionally reduce the swelling ability of PMAA regions in the PMAA/PAAm IPN.

• The ESR dependence on IPN's composition is influenced also by the interaction between both IPN's components. The hydrogen bonds formation in the case of PMAA is possible in wider pH range, including neutral pH, as compared to the PAA because PAA has pK_a ∼ 4.5 while for PMAA pK_a is around 5.5.¹⁰ That means that below pH = 5.5, PMAA could form H bonds as it was reported for e.g. PMAA interaction with poly(N-isopropylacrylamide) (PNIPAAm). There hydrogen bonds were shown to be formed between the carboxylic groups of PMAA and the amide groups of PNIPAAm.¹⁰ In a similar manner, the PMAA chains could form hydrogen bonds with PAAm along with the hydrogen bonds between themselves.

All these factors working together result into the observed ESR decrease with PMAA's content increase in the IPN of PMAA/PAAm. As all these factors are governed by the IPN's composition, it could be summarized that the PMAA/PAAm ratio is a powerful tool to control the PMAA/PAAm IPN's network density and hence their ability to swell. Translated into the expected drug delivery performance of these novel IPN, the varying of their composition would be a tool to control the drug diffusion within the networks during drug loading as well as the drug diffusion out of them during the drug release.

VPM's entrapment efficiency

The VPM entrapment efficiency (EE) as a function of IPN's composition is presented in Fig. 2. EE follows the same dependence as the ESR – the increase in PMAA content results into an EE decrease. This is not fully expected as the acidic component of IPN (PMAA) is the one which was expected to ensure the strong interaction with the cationic drug VPM. But then, the decreased swelling ability of IPN due to the hydrophobic nature of PMAA along with the lower ionization degree of PMAA at neutral pH, defined by its high pKa value, could be the reasons for this behavior.

Fig. 2 Entrapment efficiency dependence on IPN PMAA/PAAm composition.

The EE increases from 10% for IPMA1 (the IPN with the highest PMAA content) to 55% for IPMA6 and IPMA7 which have the lowest PMAA content but highest ESR among the IPN samples. The EE decreases almost linearly as PMAA content increases (regression coefficient = 0.98) which confirms that the IPN's composition defines the VPM's EE by controlling the IPN's total network density along with the enhancement of the PMAA–VPM ionic interaction.

Microhardness of PMAA/PAAm IPN

The mutual entanglements between PMAA and PAAm chains in the IPN structure is expected to influence their Vicker's microhardness (MH) as MH strongly depends on the number of physical and/or chemical junctions in polymer networks.¹¹ The MH dependence on PMAA/PAAm IPN composition is presented in Fig. 3. The dependence is “bell”-shaped with a maximum at PMAA to PAAm ratio ∼1 : 1 (IPMA3, φ^PMAA = 0.52). One could expect that as the ESR decreases, the MH should increase. This is true for IPN with lower PMAA content – up to φ^PMAA = 0.52 (Fig. 3) where the hydrogen bonds formation in IPN should be the most pronounced. At φ^PMAA > 0.52, however, the further ESR decrease results into a MH decrease. Thus, another factor besides the chains entanglements and hydrogen bonding between IPN components starts to play role when PMAA prevails in the IPN's composition. The hydrophobic clusters of compacted PMAA chains could be this factor. Naturally, the increase in PMAA content results into an increase in the number and size of these clusters.

Fig. 3 Vicker's microhardness dependence on IPN PMAA/PAAm composition (error bars are within the size of the graph points).

In addition, SN PMAA has lower MH value as compared to the SN PAAm. Thus, the increase of the former's content and its prevailing in the IPN would decrease the IPN's MH values.

In summary, ESR dependence on IPNs' composition is a result from the interplay between three factors: (i) the hydrophilic/hydrophobic balance in the IPN varied through the PAAm/PMAA ratio; (ii) the interaction between both networks, consisting in mutual penetration and interlacing as well as (iii) the hydrogen bonds formation between the IPN components.

The same factors define the MH dependence on IPN's composition however they do not act all in one direction as it is the case with ESR. Thus, the MH dependence on IPN's composition in the case when φ^PMAA > 0.52 (Fig. 3) deviates from the expected dependence due to the prevailing of the softer network (PMAA).

Morphology of IPN PMAA/PAAm – unloaded and drug loaded

The morphology of IPN obtained via the sequential method is characterized by phase separation due to the spinodal decomposition taking place during the IPN's formation.¹² In order to check if this is true for the novel PMAA/PAAm IPN, we have studied the morphology of fractured surface of IPN PMAA/PAAm with different compositions (Fig. 4).

Fig. 4 Morphology of IPN PMAA/PAAm with different compositions.

Each one from the studied IPN samples exhibits a phase separated structure consisting in small domains from the PAAm (the 2^nd network) uniformly dispersed within the PMAA matrix (the 1^st network). PAAm network is the loose one (40 times lower crosslinking agent concentration was used for its preparation as compared to PMAA) and it looks brighter in the SEM images while PMAA component has significantly denser network and thus it looks darker (Fig. 4).

Increasing the 2^nd to the 1^st network ratio (i.e. the PAAm content), results into two types of IPN PMAA/PAAm morphology:

• For φ^PMAA > 0.52 the IPN's morphology is characterized by the presence of small PAAm (the 2^nd network) domains evenly distributed within the PMAA matrix (the 1^st network). The size of these domains increases as the PAAm's content increases ranging from below 100 nm for IPMA1 (φ^PMAA = 0.86), reaching 200 nm in IPMA5 (φ^PMAA = 0.30) (Fig. 4). The phase separation is impeded by the high density of the chemical cross-linking of the 1^st network PMAA, which keeps the 2^nd network (PAAm) domains size comparatively small.

• For φ^PMAA < 0.52 an inversion of the phases occurs which is initiated by the touching between the PAAm domains at φ^PMAA ∼ 0.52 and results into the formation of a network from PAAm's interconnected cylinders at lower φ^PMAA values, i.e. higher PAAm content.¹⁵ The critical point is IPMA3 (φ^PMAA = 0.52), i.e. PAAm/PMAA ∼ 1/1 weight ratio, where also the ESR and MH dependences on IPN's composition markedly change their behavior.

Thus, the morphology of the IPN is changing as a function of the IPN's composition giving rise and being related to the composition defined change in their properties. At the φ^PMAA < 0.52, the PAAm network starts to form interconnected cylinders which could be related to the MH increase in the same region as the “harder” component PAAm starts to prevail and to form its own “matrix”. On the contrary, when the PMAA prevails (φ^PMAA > 0.52), the MH of IPN decreases also due to the IPN's morphology change where within the “softer” PMAA matrix “float” small domains from the “harder” PAAm. For similar systems it is known that the overall MH is mostly determined by the MH of the “softer” component (PMAA).¹¹ Thus, the morphology of the IPN defines and corresponds to the observed MH dependence on IPN's composition.

The IPN morphology, presented in Fig. 4, in fact resembles a nanocomposite-like structure, where nanodomains from PAAm are dispersed into PMAA matrix (φ^PMAA > 0.52). The phase separation in IPN is controlled by the PAAm to PMAA (2^nd to 1^st network) ratio.¹² This specific IPN's morphology is expected also to change the drug delivery profile, i.e. it appears to be an additional tool to control the drug diffusion in and out of the IPN. This is another parameter that would control the characteristics of the newly developed IPN as delivery vehicle for sustained drug release.

The morphology of PMAA/PAAM IPN (Fig. 4) differs from the morphology, observed for PAA/PAAm IPN.³ There, the 2^nd to the 1^st network ratio increase resulted only into the growth of the 2^nd network domains without the formation of the interconnected cylinders morphology, i.e. without an inversion of the phases to take place. The reason for this difference is the enhanced hydrophobicity of PMAA as compared to PAA.

The VPM loading in IPN PMAA/PAAm resulted into a change in their morphology. The morphology of a broken surface of IPN samples with different composition after VPM loading is shown in Fig. S2.† There, the inclusion of the drug into the polymer matrix is clearly seen. The VPM inclusions are uniformly dispersed within the IPN matrix for all IPN compositions. The IPN's morphology after VPM loading is much coarser and the fine phase separation observed for the neat IPN samples (Fig. 4) is hardly seen because the specific phase separation of IPN PMAA/PAAm is utilized for the in situ VPM deposition within the IPN samples.

Thermal properties of IPNs PAA/PAAm – unloaded and drug loaded

The thermal properties of all IPN and both SNs were studied by TM-DSC (Fig. S3†). All IPN samples show one glass transition temperature T_g which is close to the T_gs of both SNs, PMAA and PAAm (Fig. S4†). It is known from literature that both neat SNs have very close T_gs, respectively 185 and 188 °C,¹³ which differ significantly when compared to the observed in this study SNs' T_gs (Fig. S3 and S4†). This could be partially due to the low heating rate used for TM-DSC measurements (2° min⁻¹) but also to the strong plasticizing effect that water has on PMAA and PAAm (all samples studied by DSC were stored under room conditions). Similar strong plasticizing effect was reported for PAAm where T^PAAm_g decreased by 70 °C when PAAm contained ∼8 wt% water while for the poly(PAA-co-PAAm) this effect is even stronger – ∼90 °C.¹⁴

T_gs of IPN samples depend on the IPN's composition (Fig. S4†) and as φ^PMAA increases, the IPN's T_g decreases because PMAA is the component with lower T_g. The lower T_g of SN PMAA corresponds to its lower MH values (Fig. 3) as compared to the T_g and MH values of SN PAAm. The IPN's T_g dependence on composition is well described by the straight line drawn according to the additivity law (Fig. S4†).

VPM is a crystalline solid which melts at 144 °C (Fig. 5). It was interesting to check if the VPM loading resulted into change in the thermal properties of both the polymer vehicle as well as of the loaded drug. Along with the reversing heat flow for neat VPM, in Fig. 5 are presented the same for IPMA4 (φ^PMAA = 0.39) unloaded and VPM loaded. When VPM is loaded in the IPN, the drug's melting peak disappears due to the VPM amorphization. The drugs' amorphization is a way to improve their solubility and dissolution rate as well as their bioavailability.¹⁵ The drug becomes amorphous in the current case due to its interaction with the polymer matrix (IPN) and most probably with the PMAA component. This interaction is proved by the slight increase (∼1 °C) in T_g of IPMA4 after VPM loading (Fig. 5).

Fig. 5 TM DSC for IPMA4 (φ^PMAA = 0.39), IPMA4 loaded with VPM and neat VPM.

The changes in the thermal properties of both PMAA/PAAm IPN and VPM are similar to the results obtained for IPN PAA/PAAm loaded with VPM.³ There, the drug and the polymer vehicle mutually influenced their thermal properties due to the strong interaction between them.

As a result of the interaction, the IPN's T_g increased and the drug melting peak disappeared. The T_g's increase observed for the PMAA/PAAm IPN, however, is not as strong as it was reported for the PAA/PAAm IPN which could be related to the fact that PAA is a stronger acid as compared to PMAA.

ATR-IR spectroscopy of PMAA/PAAm IPN

The detected by DSC polymer–drug interaction was further studied by ATR-IR spectroscopy. The neat IPN were analyzed via ATR-IR spectroscopy (Fig. S5†) and their characteristic bands are summarized in Table S1.† In the IR spectra of the neat IPN a shift to lower wavenumbers is observed for both amide group-originating bands. This shift is due to the interaction between the amide group of PAAm and the COOH group from the PMAA. This interaction is further proved by the disappearing of the doublet at ∼1161 cm⁻¹ and ∼1236 cm⁻¹ (for υ_C–O + δ_O–H) in the IPN as compared to the neat PMAA. Also, the C=O stretching of the PMAA carboxylic group appearing at ∼1696 cm⁻¹ in the SN PMAA (Fig. S5 and Table S1†) is shifted towards lower wavenumbers in the IPN as compared to the neat PMAA, which again could be due to the participation of the respective group (C=O) in hydrogen bonds.¹³

One needs to mention here that the wavenumber decrease in the IPN PMAA/PAAm is less strong as compared to the IPN PAA/PAAm.¹⁶ The reason could be the more pronounced ionic interaction between COOH and CONH₂ groups in the latter as compared to the former due to PAA's stronger acidity (lower pKa) as compared to PMAA.

Thus, the IR study of the neat IPN PMAA/PAAm demonstrates an interaction between the PMAA and PAAm through hydrogen bonds formation between their COOH and CONH₂ groups.

In Fig. 6, the IR spectra of the neat VPM and IPMA1 (φ^PMAA = 0.86), loaded and unloaded with VPM, are presented. In the neat IPMA1 spectrum, the typical for its components PMAA (in blue) and PAAm (in red) IR bands appear.

Fig. 6 ATR-IR spectra of neat VPM and IPMA1 (φ^PMAA = 0.86) loaded and unloaded with VPM.

For IPMA1 loaded with VPM, the bands characteristic for neat VPM could be also seen, but these bands are wider as compared to the neat drug (VPM, Fig. 6) which is an indication for the VPM interaction with the polymer vehicle.¹⁰ The characteristic VPM's bands are presented in Table S2.†^17,18 The results from the IR study on the IPMA1 loaded with VPM suggest the formation of R₃NH⁺:COO⁻ ion pair¹⁹ between the VPM NH₂ and COO⁻ from PMAA. This is proved by the disappearing of the VPM's N–H stretching vibration band at ∼2800–2300 cm⁻¹ due to the protonation of the VPM's amine group when loaded into the IPN. The protonation is caused by its interaction with COO⁻ from PMAA. The IR spectra confirm the proposed at the beginning of this work ionic interaction between the polymer vehicle and the cationic drug, which was also indirectly proved by the DSC results (VPM amorphization).

From the same spectra, however, it could be concluded that the polymer–drug interaction takes place also through the formation of hydrogen bonds (Fig. 6). The C–H stretching vibrations of the methoxy groups in VPM, which appear at ∼2841 cm⁻¹ in the neat VPM, are shifted to 2838 cm⁻¹ in IPMA1, loaded with VPM, which is also an indication for drug–polymer interaction. The same interaction is the reason for the slight decrease in the wavenumber of the C=N stretching vibrations of the saturated alkyl nitrile in the neat VPM (∼2237 cm⁻¹) which appears at ∼2235 cm⁻¹ in the IPMA1. The changes in the peak shape (Fig. 6) and the bands positions (Table S2†) for the stretching vibrations of the –OCH₃ group and –C≡N group of VPM arise from the hydrogen bonding between VPM and the polymer matrix.

Thus the IR study shows that the IPN PMAA/PAAm interact with the loaded VPM via: (i) ionic interactions between COO⁻ from PMAA and R₃NH⁺ from VPM as well as (ii) via hydrogen bonds between the polymer matrix and the drug. These results confirm the role of the IPN functionality for the polymer vehicle–drug interaction and hence for the sustained drug release profiles that these IPN could ensure.

VPM's in vitro release

In vitro drug dissolution study was performed to determine the potential of the IPN PMAA/PAAm for sustained VPM release. The results are presented in Fig. 7. The SN PAAm is not suitable for sustained VPM delivery because it releases more than 85% from the loaded VPM for less than 1 hour. The reason is the high swelling ability of PAAm and its highly hydrophilic nature which allows for fast VPM loading but also for fast drug release. This lack of control is one of the hydrogels' disadvantages as drug delivery systems. The SN PMAA is also not an appropriate system for sustained VPM release, because it could release only ∼65% from the loaded VPM after 24 hours. This is most probably due to the strong drug–polymer interaction which does not allow for the release of the entire amount of the loaded drug.

Fig. 7 VPM release from IPN PMAA/PAAm, SNs PMAA and PAAm.

The VPM release profiles depend on the IPN composition. The IPMA5 shows a good potential for VPM sustained release. This IPN has EE ∼ 50%, almost linear VPM release profile between 2^nd and the 8^th hours (regression coefficient = 0.984) but releases only ∼50% of the loaded VPM most probably due to the strong ionic interaction with the cationic drug as it was the case with the SN PMAA.

The IPMA1 is the IPN with the less satisfactory performance in terms of VPM's sustained delivery. For this IPN composition, it was expected the strongest interaction polymer vehicle-cationic drug (VPM) as it has the highest PMAA content, however this sample has the lowest EE (∼10%) and shows burst effect although it releases ∼90% of the loaded VPM for 24 h.

The best performing IPN is IPMA7, i.e. the one with the lowest PMAA content. This IPN shows the highest EE (∼55%), no burst effect and nearly zero order release profile (regression coefficient = 0.990) between the 2^nd and the 8^th hours, i.e. under conditions similar to the ones in the intestines.

In summary, the best sustained drug delivery performance is ensured by the right combination of functionality and swelling ability of PMAA/PAAm IPN. The best illustration is IPMA7 where these two factors are playing together thus creating the best performing sustained drug delivery system for VPM. Thus, the concept for IPN versatile application for sustained drug delivery was successfully proved.

Conclusions

Novel IPN of PMAA and PAAm were synthesized and applied as a polymer vehicle for VPM sustained release. The IPN's network density was controlled by changing PMAA/PAAm ratio, i.e. the IPN composition. In this way, the IPN's swelling characteristics and microhardness were controlled. The two components of the newly developed IPN interact between themselves as revealed by the DSC and IR spectroscopy studies. Moreover, the IPN itself interact with the cationic drug (VPM) through ionic as well as through hydrogen bonds. As a result, VPM was amorphized after loading into the IPNs and the T_g of the IPN slightly increased.

The VPM's release profiles strongly depend on the IPNs' composition, the IPNs with prevailing PAAm showing a potential for VPM's sustained release. The IPMA7 could be considered as the most appropriate for sustained drug delivery because the balance between PMAA and PAAm enables the most precise control over the VPM release. Besides the pure functionality, the swelling ability of the polymer vehicles appears to have a strong impact on both the drug entrapment efficiency as well as on the drug release profiles.

In summary, new IPN were synthesized and characterized and their potential for sustained drug delivery of a cationic drug (VPM) was demonstrated. It was proved that the IPN's composition is a strong factor that controls the properties of the system and hence its behavior as a drug delivery system. These advantageous properties of the IPN PMAA/PAAm could be further utilized for controlled release of other drugs with different physicochemical characteristics and therapeutic requirements.

Acknowledgements

The authors acknowledge the financial support of the Bulgarian National Science Foundation, Project No DFNI ТО2/15 and ​DAAD Project DNTS/Germany 01/12.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14067j

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