Rheological properties and crystallization behavior of comb-like graft poly(L-lactide): influences of graft length and graft density

Abstract

Three series of narrowly dispersed comb-like graft poly(L-lactide) (PLLA) with controlled graft length and graft density have been facilely synthesized and characterized. The structural information was determined by means of ¹H NMR, GPC, and viscometry, demonstrating the well-defined grafted structure. The effects of grafting parameters on rheological properties, thermal and crystallization behaviors were investigated systematically to illustrate the structure–property relationships. By the analysis of the rheological behavior, the increase of graft length and graft density can both lead to an elevated value of zero-viscosity (η⁰), enhanced complex modulus (G*) and increased complex viscosity (η*). Comparatively, graft density contributes more to the improvement of rheological properties than graft length does. For thermal properties, DSC results show that the glass transition temperature (T_g), the melting temperature (T_m) and the crystallization enthalpy (ΔH_m) of graft PLLA increase with the chain length and decrease with the graft density. Furthermore, spherulite morphologies and growth rates were studied by polarized microscopy (POM), and the observation revealed a remarkable improvement of radius growth rate of the spherulites (G) when graft density increased. The results indicate that G depends more on the molecular weight of each arm (M_n,arm) than the total M_n.

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Introduction

Poly(L-lactide) (PLLA) is a biodegradable and biocompatible polymer. It combines both convenient accessibility from renewable resources and well-understood synthesis.¹ Therefore, PLLA has been studied intensely in recent years, due to its high mechanical strength, and excellent shaping and molding properties. Despite these obvious advantages, however, PLLA shows some disadvantages such as low glass transition temperature (T_g), slow crystallization, poor stability, as well as its brittle nature. To tailor the properties of PLLA for more suitable applications, many methods are performed as copolymerization, cross-linking, chain-extension, branching, stereocomplexation, blending etc. One of the promising methods is to introduce branched or grafted structure into PLLA.² By varying the molecular architecture, PLA could be useful for more applications such as surfactants,³ compatibilizer,⁴ and also as carriers for drug delivery.⁵

Up to now, different architectures such as star-like,⁶ comb-like,⁷ and other branched PLA⁸ have been successfully synthesized, with the investigation on their properties originated from their molecular architectures. As noted, the branching strongly affects the chain motion in polymer melt and determines rheological properties of polymer. The branched polymer were found to exhibit stronger shear thinning, higher Newtonian viscosity, enhanced storage modulus compared with the linear counterpart.⁹ For the crystallization behavior, some of the studies elucidated that the crystallization kinetics of the branched PLLA are inferred to be faster than the linear analog.^2a,10 C. B. Park et al. systematically explored the effect of chain branching on PLLA's crystallization kinetics.^10b The branched PLLA with up to 0.7 wt% of the given chain extender performed enhanced the degree of crystallization. Mihai et al. also reported that addition chain extender improved the crystallization and the foaming behavior of PLLA samples.¹¹ However, Hideto Tsuji et al. reported that the crystallinity (X_c) and radius growth rate of the spherulites (G) for the branched PLLA were lower than its linear counterpart.¹² Since the branched PLA have been reported with different crystallization behavior from its linear one, the effect of the branching parameters on PLA's properties is still not well-understood.

Comparatively, the case of the comb-like PLA is more complicated, as both the main chain architecture and the side chain parameters are related to PLA's performance. Zhao et al. prepared comb-like PLA using poly(2-hydroxythyl methacrylate) (PHEMA) as the backbone.¹³ Since there are numbers of hydroxyl groups on the PHEMA, it is hard to control the graft density of PLA side chains. Hillmyer and co-workers were able to synthesize a graft PLA with poly(1,5-cyclooctadiene-co-5-norbornene-2-methanol) (PCN) as the backbone.¹⁴ However, both the PCN and the resulting PLA showed broad polymer disperse index (PDI_PCN = 1.69, PDI_PLA = 1.88). The comb-like PLA is always with broadened molecular weight distributions and very uncontrollable topological structures. Due to the difficulty of preparing well-defined structure, relatively few literatures reported the effect of branching on the rheological property and crystallization behavior of graft PLLA. A systematic relationship between the main chain structure, the grafting parameters and the properties of the PLLA is unclear.

In this regard, we have recently reported the synthesis of comb-like PLLA with different backbone structures by using ‘graft-from’ technique.¹⁵ This technique requires a preliminary synthesis of linear or star-shaped polybutadiene (PB) backbone containing multiple initiator sites. By comparing the properties of linear PLLA with linear-comb and star-comb PLLA, the influence of main chain structure was well illustrated in the previous paper. However, the effect of side chain parameters was not considered in that initial study. This work aims to investigate the correlations between side chain architectures and properties of comb-like polymer. On the basis of three series of narrow dispersed comb-like PLLA with various graft length and graft density, this report focus on the effects of the side chain parameters (including graft length and graft density) on the intrinsic viscosity, rheological property, thermal and crystallization behaviors.

Experimental section

Materials

Butadiene (Yanshan Petrochem. Co., polymerization grade) was treated with little of n-butyllithium (n-BuLi) to remove the moisture and inhibitor. n-BuLi (J&K Chemical, 2.5 M solution in n-hexane), was determined by Gilman–Haubein double titration and stored under 0 °C.¹⁶ Dichloromethane (CH₂Cl₂, Aladdin), and cyclohexane were distilled from CaH₂ under nitrogen. Tetrahydrofuran was dried over sodium benzophenone ketyl under nitrogen and freshly distilled. The terminating agent 2-propanol was degassed via three freezing–evacuation–thawing cycles. L-Lactide (Jinan Daigangbio. Co., 99%) were triple recrystallized from ethyl acetate respectively, then dried and stored in a glovebox. 1,8-Diazabicyclo[5,4,0]undec-7-ene (DBU, Sigma-Aldrich, 98%) was dried over calcium hydride, distilled under reduced pressure and stored in a glovebox. Trifluoromethanesulfonic acid (TfOH, Aladdin, 98%), formic acid (HCOOH, Aladdin, 88%), hydrogen peroxide (H₂O₂, 30%), and all other reagents were used as received from the commercial source without further purification.

Instrumentation

The molecular weight and PDI of the polymers were determined by GPC using a Waters 1515 HPLC pump, a Waters 2414 refractive index detector, and PS columns (one PL gel 5 μm 10E4A and one Shodex KF805) in THF as eluent at a flow rate of 0.6 mL min⁻¹ at 35 °C calibration based on PS standards (Shodex PS STD SM-105). ¹H NMR spectra were record on a Bruker Avance 400 MHz spectrometer in CDCl₃ at 25–30 °C at a concentration of 4% w/v. The specific optical rotation, [a] of the polymer was measured in chloroform at a concentration of 1 g dL⁻¹ at 25 °C using a JASCO J-810 Polari meter at a wavelength of 589 nm. Intrinsic viscosity. The intrinsic viscosities ([η]) of PLLA were determined in CHCl₃ by a 0.4 mm Schott Ubbelohde viscometer at 30.0 °C. All samples were filtered before test. Flow time of the solvent and that of each of polymer solutions with four different concentrations below the overlap concentration c* were measured to determine [η]. [η] was estimated via the extrapolation of the relative viscosities (η_r) vs. polymer concentration plot for the synthesized polymer. The thermal behavior of the polymers were measured on differential scanning calorimeter (DSC, TA, Q520). Each sample was heated to 200 °C at a heating rate of 10 °C min⁻¹ in aluminum pans under nitrogen atmosphere. Thermal history was removed by keeping the samples at 200 °C for 3 min. Then samples were rapidly cooled to 100 °C and isothermally crystallized for 30 min, followed by cooling to 20 °C and heated to 200 °C at 10 °C min⁻¹. The crystalline morphology of the polymer was observed using a Leica DM4500P polarized optical microscopy (POM) equipped with a Linkam THMS420 hot stage. The PLLA samples were melted at 200 °C for 3 min to eliminate thermal history and then cooled to the isothermal crystallization temperature at a cooling rate of 40 °C min⁻¹. Then take the optical micrographs at appropriate times during the crystallization to examine the growth rate of PLLA spherulites. Rheological measurements were performed on a stress-controlled Rheometer AR2000 (TA Instruments Ltd.) equipped with parallel-plate geometry (diameter of 25 mm) and a gap of 1 mm at constant temperature 190 °C. Dynamic frequency sweep measurements were carried out in an oscillatory shear mode from 10 to 0.01 rad s⁻¹.

Preparation of comb-like PLLA

Synthesis of macroinitiator. Polybutadiene (PB) of defined molecular weight was synthesized by anionic polymerization of butadiene with BuLi in cyclohexane with a small amount of THF as a modifier in a flask at 50 °C for 3 h, quenched by the addition of excess methanol, and dried under vacuum. The PB product was then epoxidized and then hydroxylated. In the epoxidation step, different molar ratio of formic acid and H₂O₂ ([1,4-butadiene]₀/[HCOOH]₀/[H₂O₂]₀ = 1/0.3/0.3, 1/0.4/0.4 and 1/0.5/0.5) were used in order to produce PB with various epoxidation degree. In the hydroxylation step, epoxidized PB (PBE) in THF solution was treated with TfOH and H₂O to produce multi-functional initiator PB-OH ([epoxy group]₀/[TfOH]₀/[H₂O]₀/[THF]₀ = 1/1.5/15/30). The detailed information about the synthesis and characterization of the PB backbone could be found in our previous work.¹⁵

Synthesis of comb-like PLLA. The observed PB-OH was then used as a macroinitiator in the ring-opening polymerization (ROP) of L-lactide in CH₂Cl₂ with 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as a catalyst to form graft PLLA. The polymerization proceeded at room temperature within 1 h and was quenched by the addition of benzoic acid. The length of PLLA grafts was simply controlled by varying the feeding ratio ([LA]₀/[OH]₀ = 10, 30, 50, 70, and 100). The ¹H NMR and GPC spectrums are shown in ESI Fig. S1 and S2.† The graft PLLA is denoted by PBxPLLAy, where the x and y represent the calculated number of arms on the PB backbone and the lactide units per arm, respectively.

Results and discussion

Synthesis and characterization of comb-like PLLA

In this study, three series of linear-comb PLLA with different graft density were synthesized through ring-opening polymerization (ROP) of L-lactide, by using 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as the catalyst, hydroxylated polybutadiene (PB-OH) as the macroinitiator.

Synthesis of macroinitiator. The polymerization of butadiene in cyclohexane with a small amount of THF as a modifier yielded a polybutadiene (PB) with high 1,4-content (90%).¹⁷ The produced PB was partially epoxied only at the 1,4-PB sites.¹⁸ Double bond with higher electron density favors the epoxidation, which is relatively less sensitive to steric effect. The ratio of [1,4-butadiene]₀/[HCOOH]₀/[H₂O₂]₀ was added as 1/0.3/0.3, 1/0.4/0.4 and 1/0.5/0.5, respectively, to produce PB with different epoxidation degree. Accordingly, epoxidized PB with different epoxidation degree were successfully obtained by controlling the addition of H₂O₂ with proper stirring rate. After hydroxylation step, which was a 100% reaction of the epoxidation groups, three types of PB-OH macroinitiators with designed hydroxylation degree (10.4%, 11.7% and 16.4%, determined by ¹H NMR) were prepared. The hydroxylated PB were named PB-OH13, PB-OH18 and PB-OH25, respectively, by the number of the hydroxyl groups and listed in Table 1. The merit of this linear backbone based on anionic polymerization is its narrow molecular weight distribution (PDI ≈ 1.1) and the evenly dispersed hydroxyl groups. Graft PLLA from other linear backbone, for instance, based on linear polyglycerols or PCN, usually has broad PDI (>1.5) and uncontrollable graft density.^6a,14

Table 1 Characterization data of macroinitiators

Macroinitiator	Mn^a (g mol^−1)	PDI^a	Hydroxylation degree^b	No.OH^c
a Determined by GPC analysis with polystyrene standards.b Determined by the ^1H NMR.c The number of the hydroxyl groups on each polybutadiene chain: [No.OH] = e.g. (4200/58 × 90% × 10.4% × 2), where 58 g mol^−1 was calculated as the average molecular weight of each backbone unit.d Sample data from ref. 15.
PB13-OH^d	4200	1.1	10.4%	13.4
PB18-OH	4500	1.1	13.1%	18.3
PB25-OH	4850	1.1	16.4%	24.7

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Synthesis of comb-like PLLA. The multi-functional PB-OH were then used as macroinitiators in the ROP of L-lactide with DBU as a catalyst to produce graft PLLA. The length of PLLA grafts was simply controlled by varying the feeding ratio ([LA]/[OH] = 10, 30, 50, 70, and 100). The characterization data are summarized in Table 2.

Table 2 Characterization data of comb-like PLLA

Sample	[OH]^a/[L-LA]/[DBU]	Mn,theo^b (kg mol^−1)	Mn,NMR^c (kg mol^−1)	Mn,GPC^d (kg mol^−1)	PDI^d	[η]^e (dL g^−1)	[η ]theo^f (dL g^−1)	g′^g
a The mole of hydroxyl groups of the PB-OH.b Mn,theo = [L-LA]/[OH] × Mn,LA × No.OH + Mn,PB-OH, where Mn,LA is the molecular weight of LA and Mn,PB-OH is the molecular weight of the corresponding PB-OH.c Determined by ^1H NMR.d Determined by GPC analysis with polystyrene standards.e Measured in CHCl3 by a 0.4 mm Schott Ubbelohde viscometer at 30 °C.f Values of ηtheo were calculated by the eqn (1),^19 where K = 5.45, α = 0.73 the Mark–Houwink parameters.g Calculated by the eqn (3).h Sample data from ref. 15.
PB13PLLA10^h	1/10/0.1	23.9	24.4	25.0	1.19	0.35	0.57	0.61
PB13PLLA30^h	1/30/0.3	62.8	61.2	58.4	1.15	0.58	1.06	0.55
PB13PLLA50	1/50/0.5	100.7	98.3	97.6	1.13	0.74	1.54	0.48
PB13PLLA70^h	1/70/0.5	140.6	139.6	137.1	1.14	0.97	1.98	0.49
PB13PLLA100^h	1/100/0.5	198.9	191.5	198.6	1.16	1.20	2.59	0.46
PB18PLLA10	1/10/0.1	30.4	31.4	31.3	1.23	0.32	0.67	0.48
PB18PLLA30	1/30/0.3	82.3	78.3	76.2	1.22	0.59	1.29	0.46
PB18PLLA50	1/50/0.5	134.1	128.3	125.7	1.16	0.75	1.86	0.41
PB18PLLA70	1/70/0.5	185.9	185.1	184.0	1.17	1.01	2.45	0.41
PB25PLLA10	1/10/0.1	40.8	41.6	41.6	1.23	0.33	0.82	0.40
PB25PLLA30	1/30/0.3	112.8	107.1	104.2	1.22	0.64	1.62	0.40
PB25PLLA50	1/50/0.5	184.8	170.8	166.1	1.21	0.83	2.28	0.36
PB25PLLA70	1/70/0.5	256.8	251.5	248.6	1.19	1.12	3.05	0.37

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The ¹H NMR, GPC equipment were employed to elucidate their fine structures (Fig. S1 and S2†). As seen in the ¹H NMR spectrum of graft PLLA with assignments, the resonance of PB main chain (about 5.3–5.4 ppm) could be obviously detected when the molar ratios of LA were low. Moreover, the GPC traces exhibited narrow (PDI ≤ 1.23) and symmetrical elution peaks for all PLLA samples prepared with different graft density and side chain length. Our group recently proved that the kinetics study of the DBU catalyzed ROP reaction in this multi-functional initiator system indicated a first-order kinetic in monomer concentration.¹⁵ Thus, these ¹H NMR and GPC results unambiguously confirm the efficient initiation of the hydroxyl functional groups on the main chain.

The impact of the chain topology on the solution behavior was investigated by intrinsic viscosities measurement (Fig. S3†). Table 2 summarized the tested intrinsic viscosity ([η]) and the branching factor (g′) for the graft PLLA with different graft degree and various side chain lengths. The [η] of the graft PLLA decreased in the following order: PB13PLLA50 (0.48) ≥ PB18PLLA50 (0.41) ≥ PB25PLLA50 (0.36). The total molecular weight of the graft PLLA with same side chain length increases with the graft density increases, but exhibits a decreased intrinsic viscosity. In CHCl₃ solution, linear PLLA behaves more rod-like, graft PLLA exhibits much smaller hydrodynamic volumes.²⁰ For comb-like PLLA, both g′ and [η] further decreased with the graft degree increased. This is consistent with a more compact structure in the highly branched star-like PLLA and in the long chain branched (LCB) PLA.²¹ The PB25PLLA with the highest graft density exhibited the lowest g′ (≤0.4), which remarkably reduced the intrinsic viscosity of PLLA.

Rheological property

All the rheological measurements were carried out at 190 °C. The scale of the frequency was from 0.01 to 10 Hz. The influences of grafting parameters (graft length and graft density) on the complex modulus (G*) and complex viscosity (η*) were investigated, as defined in eqn (1) and (2):

G* = [(G′)² + (G′′)²]^1/2 (1)

where G′ is storage modulus, G′′ is loss modulus and ω is angular frequency. The influences of grafting parameters on the rheology were detailed studied in the following sections.

Effect of graft length on the rheology. Take PB18PLLA as an example, Fig. 1 shows the variation of G* (Fig. 1a) and η* (Fig. 1b) as a function of the frequency plots of PB18PLLA with varied side chain length. The complex modulus of graft PB18PLLA increase with the length of side chain, or with the total molecular weight. Besides G*, η* is also very sensitive to the side chain length. To fit the viscosity data by the cross equation written as:²²where η⁰ is the zero-shear viscosity, λ is a relaxation time which inversely accounts for the onset of shear-thinning region, and α is a shear-thinning index. For all the graft PLLA samples, a continuous shear-thinning region can be observed in the measured frequency range, without any symptom of leveling off at low shear frequencies. Therefore, the value of η⁰ can hardly be related to the Newtonian viscosity of the general linear viscoelastic model. In order to correlate the rheological property with topological structure PLLA, η⁰ can be approximated with the η* obtained at the frequency of 0.01 Hz, which is listed in Table 2. As shown in Table 2, the value of η⁰ increases from 0.79 Pa s (PB18PLLA10) to 6.37 Pa s (PB18PLLA70). The above results indicate that the increase of side chain length remarkably enhances the η* and strengthen the G* of PB18PLLA samples. Similar trends could also be observed in the PB13PLLA and PB25PLLA samples.

Fig. 1 Variation of modulus (a) and complex viscosity (b) as a function of the frequency plots of PB18PLLA with different side chain length at 190 °C.

Effect of graft density on the rheology. Fig. 2 shows the variation of G* (Fig. 2a) and η* (Fig. 2b) as a function of the frequency plots of graft PLLA with similar side chain length but varied graft density. The sample PB25PLLA30 with highest graft density exhibited the highest η⁰ (8.57 Pa s for PB25PLLA30, 0.69 Pa s for PB18PLLA30, 3.28 Pa s for PB13PLLA30, respectively, in Table 2) and more enhanced G* compared with PB18PLLA30 and PB13PLLA30. Similar results can also be obtained from the comparison between PB13PLLA10, PB18PLLA10 and PB25PLLA10. Therefore, the rheological property of graft PLLA samples with same side chain length can be enhanced through increasing the graft density.

Fig. 2 Variation of modulus (a) and complex viscosity (b) as a function of the frequency plots of different graft density PLLA with same side chain length.

Comparison of the effects of graft length and graft density on the rheology. Due to the above experimental results, the increase of graft length or graft density can lead to an enhancement of rheological properties. Other copolymers with similar graft structure exhibited the same regulation.^2b,21b,23 However, the comparison of the effects of the two grafting parameters on the rheology was seldom studied, which was ascribed to the difficulty of preparing narrow dispersed comb-like PLLA samples. To further distinguish the influence of grafting parameters of comb-like PLLA, well-defined samples were synthesized and rheological measured. Fig. 3 shows the variation of G* (Fig. 3a) and η* (Fig. 3b) as a function of the frequency plots of graft PLLA with similar molecular weight but apparently different graft density and graft length. The graft density of PB25PLLA50 is about 2 times as that of PB13PLLA100, and the side chain length of the former is only half of that of the latter. As shown in Table 2, PB25PLLA50 exhibits a three times higher value of η⁰ than PB13PLLA100 (11.57 Pa s for PB25PLLA50 and 3.07 Pa s for PB13PLLA100). Besides, the G* and η* values of the samples followed the order PB13PLLA100 < PB18PLLA70 < PB25PLLA50 through all frequencies, in parallel with their graft densities, which indicates an increased rheological property. This result reveals that the graft density has more significant influence on rheological property of the graft PLLA compared to the graft length in this system.

Fig. 3 Variation of modulus (a) and complex viscosity (b) as a function of the frequency plots of different graft density PLLA with similar molecular weight.

Crystallization behavior

Thermal property. Due to the slow crystallization of PLLA, no crystallization peak can be observed during the nonisothermal quenching process (under 10 °C min⁻¹ cooling rate, Fig. S4†). To better study the graft density effect on the thermal behavior of graft PLLA, after removing thermal history at 200 °C, samples were rapidly cooled to 100 °C and isothermally crystallized for 30 min. Fig. 4 shows the second heating DSC curves of similar molecular weight comb-like PLLA with different graft density. From the data summarized in Table 3, T_g decreases as the following order: PB13PLLA70 (51.2 °C) > PB18PLLA50 (49.4 °C) > PB25PLLA30 (46.8 °C). There are many parameters that influence glass transition temperature when chain structure changed.^10c By changing the graft density of the comb-like PLLA, two factors are altered. One is the free volume, which increases by the higher graft density gives more flexible chain ends, contributes to decrease in T_g. The other is the segmental mobility, which decreases by chains are much closer and restricted in movement, contributes to increase in T_g. Both directions in T_g between branched PLA and linear PLA were reported by Sakamoto and Tsuji.²⁴ Therefore, T_g may changes with the degree of graft changes depending on which of the two counteracting parameters controls. Here, the T_g decreases with graft density increases. The behavior of graft polymer is consistent with additional free volume from the increase in chain end.

Fig. 4 DSC curves of similar molecular weight comb-like PLLA with different graft density: (a) PB13PLLA70, (b) PB18PLLA50, and (c) PB25PLLA30.

Table 3 Thermal properties of comb-like PLLA

Sample	Tg (°C)	Tm (°C)	ΔHm^a (J g^−1)	Xc^a (%)	G120 °C^b (μm min^−1)
a Determined from the DSC curve for the second heating. Xc (%) = ΔHm/ΔHm0 × 100, the fusion enthalpy of 100% PLLA crystalline (ΔHm0) was assumed to be 93.7 J g^−1.b G radius growth rate of spherulites: measured by POM at 120 °C.
PB13PLLA70	51.2	151.9	39.8	42.5	0.71
PB18PLLA50	49.4	149.6	33.4	35.6	0.97
PB25PLLA30	46.8	145.5	25.4	27.1	1.31

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It is noticed in Fig. 4, samples displayed double melting peaks, which is due to the melt-reorganization.²⁵ The melting temperature (T_m), crystallization enthalpy (ΔH_m) and the relative degree of crystallinity (X_c) were summarized in Table 3. T_m and ΔH_m of the graft PLLA decrease with the graft density increases (PB13PLLA70 > PB18PLLA50 > PB25PLLA30). Similar trend has been reported in some other graft polymers.^2b,21a,26 The branching and grafting architecture usually delay and disturb the crystallization of polymer, due to the lower degree of molecular symmetry.^24,27 Moreover, the mobility and reputation ability of PLA chains are restrained by the graft structure, which hinders the crystallization process and makes a contribution to the imperfection. As a result, the T_g, T_m, ΔH_m and X_c of graft PLLA all decrease with the graft density increases.

Spherulitic morphology and growth rate. Spherulitic morphology and spherulite growth rate (G) of comb-like PLLAs were investigated by optical microscopy under cross-polar conditions in isothermal mode. In the typical polarization photomicrographs of graft PLLA (Fig. 5 and 6), normal spherulites were formed in order with typical Maltese-cross patterns. Therefore, the graft structure does not cause obvious structural defects during the crystal growth in this system.

Fig. 5 Selected POM micrographs during isothermal crystallization at 125 °C. (A) PB13PLLA10, (B) PB18PLLA10, (C) PB25PLLA10. The scale bar in the bottom micrograph of panel A represents 50 μm and applies to all the micrographs.

Fig. 6 Selected POM micrographs during isothermal crystallization at 120 °C. (A) PB13PLLA70, (B) PB18PLLA50, (C) PB25PLLA30.

Effect of graft length on the spherulite growth rate. The G values of PB13PLLA, PB18PLLA and PB25PLLA plotted as a function of crystallization temperature (T_c) in Fig. 7a–c, respectively. Since the crystallization of PLLA under lower T_c gives higher nucleation density, the significant impingement of spherulites occurs when temperature lower than 100 °C. G values of each graft PLLA were measured at T_c ≥ 100 °C, every point per 5 °C. The G value of PB13PLLA at 110 °C, for instance, the radius growth rate of the PB13PLLA spherulites decreased in the following order: PB13PLLA10 (2.71 μm min⁻¹) ≥ PB13PLLA30 (1.30 μm min⁻¹) ≥ PB13PLLA50 (0.97 μm min⁻) ≥ PB13PLLA70 (0.65 μm min⁻¹) ≥ PB13PLLA50 (0.35 μm min⁻¹). In Fig. 7, the spherulite growth rate, of all the three series of graft PLLA (PB13PLLA, PB18PLLA and PB25PLLA, M_n > 25.0 kg mol⁻¹) at all tested temperature, decreases with the side chain length increases due to the weakened chain mobility. The decreasing G is very common among ordinary synthetic polymers,²⁸ and also essentially consistent with the results reported by Pennings and co-workers.²⁹ However, Pengju Pan et al. reported that increasing the chain length of the side chains of the graft PLLA resulted in a higher crystallization rate³⁰ (DP of LA in the graft polymer was less than 105. The molecular weight of PLLA content was less than 15.1 kg mol⁻¹). The very different results also occurred in linear PLLAs. Sheng Xiang et al. recently investigated the effect of M_n on the crystallization rate of the linear PLLA.³¹ They demonstrated that: when M_n ≤ 18.6 kg mol⁻¹, G increased with M_n increased; when M_n ≥ 18.6 kg mol⁻¹, G decreased with M_n increased. On this point, the interesting regulation between molecular weight and crystallization rate of PLLA is consistent for both linear PLLA and graft PLLA.

Fig. 7 Radius growth rate of spherulites (G) versus crystallization temperature (T_c) for (a) PB13PLLA, (b) PB18PLLA, and (c) PB25PLLA.

Effect of graft density on the spherulite growth rate. The graft density of the polymer also plays a significant role in the process of crystallization. Take the isothermal crystallization of PB13PLLA10, PB18PLLA10, and PB25PLLA10 under 125 °C as an example (Fig. 5). The graft PLLA with different graft density but same molecular weight per arm (M_n,arm) exhibited the similar G values (around 2.2 μm min⁻¹), almost the same spherulite size under the same time. The decreasing of spherulite growth by enhanced total molecular weight was balanced with the increased density of the terminal groups. As seen in Fig. 7, the G values of three series of PLLA with same side chain length ordered as the following: PB13PLLA10 ≈ PB18PLLA10 ≈ PB25PLLA10 ≥ PB13PLLA30 ≈ PB18PLLA30 ≈ PB25PLLA30 ≥ PB13PLLA50 ≈ PB18PLLA50 ≈ PB25PLLA50 ≥ PB13PLLA70 ≈ PB18PLLA70 ≈ PB25PLLA70. This finding may be ascribed to the fact that the main factor determining G is not M_n but M_n,arm.
Comparison of the effects of graft length and graft density on the spherulite growth rate. Fig. 6 shows graft PLLA with similar total molecular weight but different graft density under 120 °C. The G value of graft PLLA spherulites at 120 °C increased in the following order: PB13PLLA70 (0.71 μm min⁻¹) ≤ PB18PLLA50 (0.97 μm min⁻¹) ≤ PB25PLLA30 (1.31 μm min⁻¹). The enhancement of spherulite growth could be explained by reduced side chain length through increased graft density. Under the similar molecular weight, more side chains means more chain ends and much shorter chain length. In some reports, PLAs with branch or graft structures can be appropriate for fast crystallization during cooling, because the branch or graft points can act as nucleating agents to accelerate crystallization, but the perfection of the resulting crystal would be disturbed as a consequence.^10b,32 However, in this work the imperfection of the crystal did not appear on the graphs of spherulites. At least, there is no obvious difference between the 13–25 arms PLLAs. Hideto Tsuji et al. reported that, compared with 3 arms PLLA, the multi-arm PLLA showed different thermal stability against star-shaped PLLA.³³ In such hyperbranched PLLA (16–21 arms PLLA), the effect of increased density of the terminal groups must be more remarkable compared with that in few-arm PLLA and the branching will not reduce the chain mobility.³⁴

Conclusions

The well-defined narrow dispersed comb-like PLLA with various graft length and graft density has been achieved. The graft density was controlled through changing the oxidation degree of the PB backbone. The grafted side chain length was controlled by the ratio of monomer and initiator. All the initiation sites on the macroinitiator seem to react under room temperature during the ROP of L-lactide, which is strongly evidenced by ¹H NMR and GPC results. Three sets of PLLA samples (M_n range from 25.0 kg mol⁻¹ to 248.6 kg mol⁻¹, PDI < 1.23) with different graft densities (13, 18 and 25 arms per main chain) and side chain length (10–100 lactide units per arm) were prepared efficiently. The correlations between the grafting parameters and the properties of the graft PLLA were systematically investigated. The increases of graft length and graft density both lead to an elevated value of zero-viscosity η⁰, enhanced complex modulus G* and increased complex viscosity η*. Comparatively, graft density contributes more to the improvement of rheological property than graft length does. As a result, the graft parameters have an obvious influence on the rheological property. DSC results indicated that the increase of graft density resulted in a lower T_g, and also the less perfect crystals that were responsible for the lower T_m and ΔH_m values observed experimentally. From the isothermal crystallization, the increase of graft density exhibits an improvement of spherulitic growth rate G, while the increase of chain length reduces G. The value of G at 120 °C was found to increase from 0.71 μm min⁻¹ for PB13PLLA70 with 13 arms to 1.31 μm min⁻¹ for PB25PLLA30 with 25 arms. Moreover, the results indicate that the main factor determining G is not M_n but M_n,arm. In other words, rheological and crystallization properties of comb-like graft PLLA were correlated to the side chain structures. The information generated would help develop structure–property relationships, which could better illustrate the synthesis and the application of the comb-like PLLA.

Acknowledgements

This work was financially supported by National Program on Key Basic Research Program of China (973 Program No. 2015CB654700 (2015CB654701)) and National Science Foundation of China (No. 21174021).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02697d

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