Combining block copolymers and hydrogen bonding for poly(lactide) toughening

Abstract

A series of ABA triblock copolymers of poly(D,L-lactide)-b-poly(butadiene)-b-poly(D,L-lactide), LBL, and their 2-ureido-4[1H]-pyrimidinone (UPy) end-functionalized analogue, UPy-LBL, were synthesized. Commercially available hydroxy-terminated poly(butadiene) (HTPB) was used as the macroinitiator for ring-opening polymerizations of D,L-lactide. UPy hydrogen bonding dimerization in solution was confirmed by ¹H NMR spectroscopy and by the thermo-reversible properties of this interaction. Mechanical properties of this polymer series revealed that only the incorporation of low molar mass PB did not significantly alter the brittle character of PLA. However, end-functionalization of the triblock copolymers with the UPy moiety effectively increased the strain at break of PLA, producing polymers with average ultimate elongations up to 58%. A relationship between UPy content and mechanical properties was established. The series of polymers with UPy functionalities ranging from 0–2 was tougher than neat PLA; the hemitelechelic polymer was more than four times tougher. To study the physical aging of these systems, uniaxial tensile-tests and DSC experiments were performed. DSC thermograms showed increases in both the glass transition temperature and enthalpy of relaxation as a function of PLA aging time. Tensile experiments demonstrated that UPy-functionalization effectively delayed aging as ductile behavior was only lost after annealing the samples for five days at 40 °C.

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Introduction

The need for degradable polymers from renewable resources is emerging as petroleum-based polymers become cost-ineffective and unsustainable. One of the most promising bioplastics is polylactide (PLA), which has properties comparable to polystyrene (PS) and poly(ethylene terephthalate) (PET).¹ However, PLA's practical applications are often limited by its inherently brittle nature. Blending strategies to toughen PLA through integration of rubber particles that serve as stress concentrators and allow for ductile failure have been accomplished with a variety of elastomers such as poly(ε-caprolactone), low-density polyethylene, and polyisoprene.^1–3

The synthesis of PLA copolymers that contain rubbery segments is another effective toughening methodology. The copolymer connectivity (e.g., random, block, graft) and architecture (e.g., hyperbranch, star shape, dendrimer) can lead to materials with drastically improved mechanical properties.¹ For instance, a graft copolymer system incorporating 5 wt% polybutadiene (PB) in the polymer backbone effectively increased PLA elongation at break by more than 200%.⁴ Similarly, the mechanical properties of linear copolymers of PLA with glycolide, caprolactone, and ε-caprolactone, directly depends on the elastomeric content.^5–7 Triblock copolymers of PLA with PB in poly(lactide)-b-poly(butadiene)-b-poly(lactide) systems have been reported to significantly increase toughness with only 8 wt% PB (M_n = 15.6 kg mol⁻¹) in the backbone.⁸ Polymer blends of PLA with a similar triblock system exhibited a 27-fold increase in the fracture strain of PLA when 10 wt% of the triblock copolymer was used.⁹

Non-covalent interactions (e.g., ionic, metal coordination, hydrogen bonding) are widely used to improve the mechanical performance of materials. Hydrogen bonding interactions are thermoreversible and allow for property tuning by heating the material above the hydrogen bond dissociation temperature.¹⁰ Among hydrogen bonding systems, 2-ureido-4[1H]-pyrimidinone (UPy) has one of the strongest self-dimerization constants (6 × 10⁷ M⁻¹ in CDCl₃ or 6 × 10⁸ M⁻¹ in toluene at 25 °C) as a result of four hydrogen bonding interactions.^11–13 The UPy moiety has been introduced into many polymeric systems to increase interfacial adhesion in polymer blends,¹⁴ increase the strength of rubber-like materials like polyisoprene or oligomeric poly(trimethylcarbonate),^15–17 and to study virtual molar mass increases in oligomeric systems of low-density polyethylene.¹⁸ However, there have only been a few examples reporting the incorporation of such hydrogen bonding motifs into glassy materials.^19–21 As a relevant example, Arrigui et al. demonstrated that the segmental mobility in glassy polystyrene is restricted in the presence of hydrogen-bonding motifs. This restriction decreases the physical aging rate and preserves the mechanical properties of the material for a longer period of time.²²

Here we report the synthesis and characterization of symmetric ABA triblock copolymers (A = poly(D,L-lactide) and B = polybutadiene, PB), and the impact of UPy end-functionalization on their mechanical properties with the aim of marrying the block copolymer and hydrogen bonding strategies in an effort to improve the mechanical performance of PLA. Through systematic studies of this system, we determined that enhancement of the toughness is correlated to the level of incorporation of UPy end groups.

Results

HTPB (hydroxy-terminated polybutadiene) with 65% of 1,2-addition content was used to initiate the ring-opening transesterification polymerization (ROTEP) of D,L-lactide to form the LBL triblock copolymers containing between 82 and 98 wt% PLA that corresponds to molar masses from 11 to 85 kg mol⁻¹ in each block (Scheme 1).²³ This series of polymers showed glass transition temperatures that increased from 49 to 56 °C as the molar mass of the PLA increased in the LBL triblocks (Table 1). The glass transition temperature of PB was only observed in the lowest molar mass samples (82–84 PLA wt%), consistent with microphase separation between these two polymers.

Scheme 1 Synthesis of hydroxy-terminated block copolymers (LBL) and UPy-functionalized block copolymers (UPy-LBL).

Table 1 Molecular characteristics of UPy- and non-UPy-functionalized triblock copolymers

Polymer [Sn(Oct)2]	Mn (NMR)^a kg mol^−1	Mn (SEC)^b kg mol^−1	Đ	Tg PB (°C)	Tg PLA (°C)	wPLA^c (%)	F^d	wUPy^e (%)	D^f (nm)
a Determined from ^1H NMR spectroscopy by comparing the relative integration of repeat unit signals to PB-terminal methylene units.b Values obtained by RI detector using polystyrene standards in CDCl3.c Calculated from the integral ratio of DPPLA and DPPB.d Determined by the ratio between the PLA terminal methine (4.38 ppm) and the CH2–NH(CO) protons from UPy-functional groups (3.3–3.5 ppm).e Determined by the ratio between the UPy molar mass and the total molar mass of the UPy-polymer times the functionality.f Domain spacing determined from SAXS at room temperature based on the principal scattering peak using D* = 2π/q*.g The Tg of PB block was not observed in these samples.h Polymer is not soluble in common organic solvents.
Non-UPy-functionalized polymers
LBL(11-3.3-11)	26	32	1.2	−47	49	84			15
LBL(17-3.3-17)	37	60	1.4	—^g	53	89			17
LBL(36-3.3-36)	76	103	1.2	—^g	56	94			20
LBL(58-3.3-58)	119	142	1.6	—^g	57	96			22
LBL(80-3.3-80)	164	190	1.6	—^g	56	97			25
LBL(85-3.3-85)	173	211	1.5	—^g	56	98			27
PLA	60	82	1.6		55	100
UPy-functionalized polymers
UPy-LBL(10-3.3-10)	24	33	1.4	−42	50	82	2.0	1.9	14
UPy-LBL(20-3.3-20)	43	54	1.7	—^g	52	90	2.0	1.1	17
UPy-LBL(36-3.3-36)	75	109	1.6	—^g	54	94	1.5	0.6	20
UPy-LBL(57-3.3-57)	117	146	1.7	—^g	55	96	2.0	0.4	22
UPy-LBL(80-3.3-80)	167	173	1.6	—^g	53	97	2.0	0.3	25
UPy-LBL(86-3.3-86)	175	205	1.5	—^g	56	98	1.8	0.3	28
HTPB	3.3	5.3	1.1	−46		0
UPy-PB	—^h	—^h	—^h	−38		0	1.7	13.6
UPy-PLA	59	81	1.5		52	100	1.8	0.5

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These series of polymers were then end-functionalized by the addition of UPy-NCO, a method previously used for hydroxy-terminated polymers.^17,18 These UPy-functionalized analogue showed very similar molar masses, glass transition temperatures, and dispersities (Table 1) which indicated that no significant polymer degradation occurred during the UPy-functionalization. The hydrogen bonding within UPy units in solution was analyzed by the presence of the characteristic UPy N–H resonances at 13.28, 11.95, and 10.18 ppm in the ¹H NMR spectra in toluene (Fig. S1†). The two resonance peaks observed are caused by the presence of 4[1H]-pyrimidinone and pyrimidin-4-ol dimers existing in solution.^12,24,25 Variable temperature ¹H NMR experiments showed that these N–H resonances became broader and shifted upfield as the temperature increased, weakening the hydrogen bonding (Fig. S2 and S3†). Sample dilution did not result in chemical shift differences providing additional confirmation of dimer stability. UPy dimerization was further verified by adding DMSO-d₆ to the solution. A significant upfield shift was observed, indicating the dissociation of UPy units by the formation of tautomer III (Fig. S4†).²⁶

LBL polymers are denoted with the respective molar mass of each block copolymer in parenthesis. For instance, LBL(11-3.3-11) represents a sample with poly(D,L-lactide) of 11 kg mol⁻¹ segments flanking a polybutadiene core of 3.3 kg mol⁻¹. UPy-functionalized analogue are denoted with the prefix UPy.

We quantified the composition in the triblock copolymers by ¹H NMR spectroscopy using the ratio between the degree of polymerization (DP) of PLA and the sum of DPs for PLA and PB (eqn S2†). The presence of three new N–H resonances at 13.28, 11.95 and 10.18 ppm, characteristic of the dimerized UPy moiety in solution (Fig. S1†),^24,25 and the upfield shift of the PLA terminal methine due to urethane formation both confirmed UPy end-functionalization. We quantified the extent of UPy-functionalization by the ratio between the methylenes attached to the urethane/urea groups from UPy (observed at 3.05–3.45 ppm) and hydroxy-terminated PLA (analyzed at 4.38 ppm) as described in eqn S5.†

The UPy-dimerization of the triblocks in solution was confirmed by the presence of the N–H resonances at 13.95, 12.06, and 10.23 ppm (Fig. S1 and S2†).¹² The solution was heated to 95 °C to analyze the dissociation temperature. Although these resonances were still present up to this temperature, they became less intense and broader. To determine the dimerization constant and the dimer stability, we studied different dilutions in toluene-d₈ (from 0.5–4 mM). However, no chemical shift change was observed. To break the hydrogen bonding associated with the UPy, DMSO-d₆ (0.1 mL) was added to a chloroform-d solution (1.8 mM), changing the chemical shifts to 11.5, 9.5, and 6.7 ppm.

Thermal analysis of these polymers obtained by DSC showed their amorphous character with comparable glass transition temperatures (55 °C). This result is expected considering that this series of polymers have almost identical molar masses (∼160 kg mol⁻¹). The morphology and domain sizes of LBL and UPy-LBL materials were characterized by small-angle X-ray scattering (SAXS) in Fig. S5.† The principal domain spacing values were obtained from: D* = 2π/q*, and are summarized in Table 1. The correlation between D* as a function of PLA content (w_PLA) is displayed in Fig. S6.†,²⁷

Mechanical properties as a function of the molar mass of triblock copolymers

The mechanical behavior of the LBL and UPy-LBL triblock copolymers was characterized by uniaxial extension of compression-molded samples that were aged for 48 h at 25 °C in a vial filled with DRIERITE® prior to testing. As the hydroxy- and UPy-functionalization could play a main role on the mechanical properties, PLA and UPy-PLA polymer controls were also synthesized from two different initiators: benzyl alcohol (for a hemitelechelic system) and 1,4-diphenylene dimethanol (for a telechelic systems). The mechanical properties of LBL, UPy-LBL and PLA are summarized in Table 2. The tensile toughness values, as a function w_PLA, are represented on Fig. 1 and stress–strain curves for representative samples are displayed on Fig. S7 and S8.†

Table 2 Mechanical properties of LBL and UPy-LBL triblock copolymers

Polymer [Sn(Oct)2]	σTS (MPa)	εb (%)	E (GPa)	TT (MJ m^−3)
Non-UPy polymers
LBL(11-3.3-11)	30.6 ± 1.0	5.3 ± 1.0	1.0 ± 0.2	0.8 ± 0.3
LBL(17-3.3-17)	40.4 ± 0.9	12.7 ± 3.2	1.6 ± 0.1	5.1 ± 2.5
LBL(36-3.3-36)	47.0 ± 0.6	13.1 ± 3.2	1.4 ± 0.7	5.1 ± 1.7
LBL(58-3.3-58)	54.3 ± 4.6	13.2 ± 2.8	1.6 ± 0.3	5.7 ± 1.6
LBL(80-3.3-80)	43.0 ± 1.8	14.8 ± 4.6	1.9 ± 0.1	5.8 ± 2.0
LBL(85-3.3-85)	45.2 ± 2.0	13.0 ± 1.4	1.8 ± 0.7	5.1 ± 2.0
PLA	59.4 ± 4.3	8.0 ± 1.4	2.1 ± 0.1	3.3 ± 1.0
UPy-functionalized polymers
UPy-LBL(10-3.3-10)	42.2 ± 5.5	5.3 ± 0.5	1.3 ± 0.1	0.9 ± 0.1
UPy-LBL(20-3.3-20)	42.8 ± 3.4	8.1 ± 3.0	1.4 ± 0.1	2.9 ± 1.3
UPy-LBL(36-3.3-36)	44.1 ± 3.0	12.8 ± 2.1	1.7 ± 0.5	4.7 ± 1.0
UPy-LBL(57-3.3-57)	42.2 ± 2.4	16.8 ± 2.8	1.6 ± 0.1	6.2 ± 0.9
UPy-LBL(80-3.3-80)	47.2 ± 1.3	20.4 ± 6.6	2.1 ± 0.1	9.3 ± 3.2
UPy-LBL(86-3.3-86)	44.2 ± 5.1	19.6 ± 4.2	1.8 ± 0.2	8.1 ± 2.1
UPy-PLA	54.4 ± 7.5	7.9 ± 1.6	2.3 ± 0.4	3.2 ± 1.2

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Fig. 1 Tensile toughness values for LBL (black) and UPy-LBL (red) triblock copolymers as a function on the PLA content.

Mechanical properties as a function of UPy-functionalization

To study the effect of the UPy units in the mechanical properties of the triblock copolymers, we synthesized a series of polymers with different levels of UPy-functionalization but same polymer composition as UPy-LBL(80-3.3-80), the toughest polymer we achieved in the telechelic system. To obtain better control over the amount of UPy incorporated, TBD was used as the catalyst instead of Sn(Oct)₂. The functionality increased with reaction time and the highest functionality obtained was 1.0 (hemitelechelics) after 24 h of reaction. This series of polymers was compared with the polymers obtained using Sn(Oct)₂ as catalyst, where the functionality obtained was 2.0 (>99% efficiency in 15 minutes). The fractional functionalization is shown in the sample name after the polymer composition. Polymer molar mass was characterized by ¹H NMR spectroscopy and SEC and found to be similar across the series. The UPy-functionality was characterized by ¹H NMR spectroscopy and DSC as described for the first series of polymers. Molar mass, dispersity, and functionality data are shown in Table 3. Tensile testing results are summarized in Table 4. The tensile toughness values, as a function of UPy-functionality in the polymer, are represented in Fig. 2, and the stress–strain curves are displayed on Fig. S9.†

Table 3 Molecular characteristics of triblock copolymers with varying UPy functionality

Polymer	Catalyst	t (h)	F^a	Mn (NMR)^b kg mol^−1	Mn (SEC)^c kg mol^−1	Đ	Tg PLA (°C)	wPLA^d (%)
a Determined by the ratio between the PLA terminal methine (4.38 ppm) and the CH2–NH(CO) protons from UPy-functional groups (3.3–3.5 ppm).b Determined from ^1H NMR spectroscopy by comparing the relative integration of repeat unit signals to PB terminal methylene units.c Values obtained by RI detector using polystyrene standards in CHCl3.d Calculated from the integral ratio of PLA and PB.
UPy-LBL(80-3.3-80)-0	TBD	0	0	161	190	1.5	54	97
UPy-LBL(80-3.3-80)-0.2	TBD	3	0.2	152	191	1.6	56	97
UPy-LBL(80-3.3-80)-0.3	TBD	6	0.3	158	190	1.5	56	97
UPy-LBL(80-3.3-80)-0.4	TBD	9	0.4	162	192	1.4	56	97
UPy-LBL(80-3.3-80)-0.6	TBD	12	0.6	157	200	1.5	56	97
UPy-LBL(80-3.3-80)-1.0	TBD	24	1.0	156	199	1.6	56	97
UPy-LBL(80-3.3-80)-2.0	Sn(Oct)2	0.25	2.0	168	216	1.6	53	98

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Table 4 Mechanical properties of triblock copolymers varying the amount of UPy-incorporated into the block copolymers

Polymer	σTS (MPa)	εb (%)	E (GPa)	TT (MJ m^−3)
UPy-LBL(80-3.3-80)-0	43.0 ± 1.8	14.8 ± 4.5	1.9 ± 0.1	5.7 ± 2.0
UPy-LBL(80-3.3-80)-0.2	44.9 ± 2.7	11.3 ± 3.8	1.8 ± 0.2	4.3 ± 1.7
UPy-LBL(80-3.3-80)-0.3	54.5 ± 1.9	15.7 ± 3.6	2.1 ± 0.1	6.1 ± 1.8
UPy-LBL(80-3.3-80)-0.4	43.0 ± 2.6	18.4 ± 5.4	2.2 ± 0.2	6.8 ± 1.8
UPy-LBL(80-3.3-80)-0.6	28.8 ± 2.5	40.2 ± 6.7	2.1 ± 0.2	8.9 ± 3.1
UPy-LBL(80-3.3-80)-1.0	25.8 ± 2.2	57.9 ± 14	1.9 ± 0.1	14 ± 3.4
UPy-LBL(80-3.3-80)-2.0	47.2 ± 2.2	20.4 ± 6.6	1.6 ± 0.1	9.3 ± 3.3
PLA	59.4 ± 4.2	8.0 ± 1.4	2.1 ± 0.1	3.3 ± 1.0
UPy-PLA	54.4 ± 7.5	7.9 ± 1.6	2.3 ± 0.4	3.2 ± 1.2

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Fig. 2 Tensile toughness values as a function of UPy-functionalization on the UPy-LBL(80-3.3-80) series of triblock copolymers (black) compared with the PLA, UPy-PLA homopolymers (red).

Scanning electron microscopy (SEM) was employed to help understand the deformation mechanisms of the UPy-functionalized materials. Hence, LBL and UPy-LBL samples were stretched until fracture, and two different regions of the tensile bar were imaged using SEM. Fig. 3(1a)–(c) illustrates the representative micrographs of the unstretched region with a smooth, defect free surface. Fig. 3(2a)–(c) illustrates the highly stretched region where crazes perpendicular to the elongation axis in the UPy-functionalized materials are evident compared with the non-UPy analogue (Fig. 3(3a)–(c)).

Fig. 3 SEM micrographs of fractured tensile bars of unstretched UPy-LBL(80-3.3-80)-1.0 (1a–c); elongated UPy-LBL(80-3.3-80)-1.0 (2a–c); and elongated LBL(80-3.3-80) (3a–c). Arrows represent the stretching direction.

The physical aging of the ductile UPy-LBL(80-3.3-80)-1.0 was followed by mechanical and thermal experiments. Stress–strain experiments employed dog-bone shaped tensile bars obtained after compression molding the polymer, which was quenched at 35 °C min⁻¹ and annealed at 40 °C under reduced pressure. Samples were annealed at this temperature to be consistent with previous PLA studies and to accelerate the aging process.²⁸ The mechanical measurements were performed after annealing times (Fig. 4). The enthalpy relaxation and changes in the glass transition temperatures of ductile UPy-LBL(80-3.3-80)-1.0 was followed by DSC. The samples received the same thermal treatment as the aforementioned tensile bars. The DSC thermograms (Fig. S10†) exhibited endothermic peaks and glass transition temperatures that both increased as a function of the annealing time.

Fig. 4 Tensile stress–strain curves of polymer UPy-LBL(80-3.3-80)-1.0 aged at 40 °C under indicated annealing time (0–5 days).

Discussion

The morphology of the LBL and UPy-LBL triblock copolymers was characterized by their one-dimensional SAXS patterns. The sharp principal peak (q*) at low scattering angle evidenced the microphase separation between PB and PLA (Fig. S5†). Low molar mass samples exhibited characteristic reflections that were consistent with hexagonally packed cylinders (C), while the higher molar mass polymers exhibited ill-resolved higher order reflections. These findings were consistent with similar triblock LBL systems, where lamellar structures were obtained at 69 wt% PLA, hexagonal packed cylinders from 78–83 wt% PLA, and spheres at 92 wt% PLA.^8,23 The principal domain spacing (D*) showed a characteristic dependence on molar mass (Fig. S6†). The lowest molar mass triblock polymer with 82 wt% of PLA, LBL(11-3.3-11), exhibited the lowest D-spacing value of 15 nm, while the highest molar mass, with 98 wt% of PLA, LBL(85-3.3-85), gave a value of 27 nm (Table 1).⁸ The one-dimensional SAXS patterns of the UPy-LBL series of polymers exhibited very similar features and domain sizes as the LBL systems, suggesting that morphology of the polymers was not altered by the end-functionalization, in agreement with previous reports.²⁹

Mechanical properties as a function of the molar mass of triblock copolymers

PLA and UPy-PLA polymers served as control samples with respect to the LBL and UPy-LBL synthesized series of materials. The neat PLA sample failed after minimal deformation with local stress whitening and without necking (Fig. S7†), corresponding to brittle behavior as expected (Table 2). The UPy end-functionalization of PLA did not alter its mechanical properties. The LBL triblock copolymers exhibited somewhat brittle characteristics but with slightly lower tensile strengths (σ_TS) than the PLA control.⁴ The tensile strength was dependent on overall molar mass increasing from 30.6 ± 1.0 to 45.2 ± 2.0 MPa as the overall molar mass increased from 26 to 173 kg mol⁻¹. The lack of significant toughening by the addition of PB into the triblock copolymer can be explained by the low PB molar mass (M_n: 3.3 kg mol⁻¹) where the rubbery domain was likely too small to effectively cavitate and prevent premature crack growth.³⁰ This is in contrast with previous reports where higher molar mass PB effectively toughens PLA.^8,9

The mechanical properties of UPy-LBL triblock copolymers showed a molar mass dependence (Table 2). UPy-LBL copolymers with a molar mass lower than 100 kg mol⁻¹ and lower than 0.6 wt% UPy (see Table 1), exhibited brittle character with similar tensile strengths to their non-functionalized counterparts. However, high molar mass materials like UPy-LBL(80-3.3-80) and UPy-LBL(86-3.3-86) with ∼0.2 wt% UPy exhibited stress whitening and neck formation. This ductile behavior resulted in a 2.6-fold increase in the ultimate elongation at break (ε_b) compared with neat PLA (20.4 ± 6.6 vs. 8.0 ± 1.4%). The overall tensile modulus (E) remained constant for all samples. Fig. 1 depicts the relationship between the tensile toughness of each series as a function of the PLA content. UPy-LBL with higher than 96 wt% PLA (Table 1) exhibited increased toughness compared to their LBL counterparts. Specifically, UPy-LBL(80-3.3-80) was 2.8-fold tougher than neat PLA (9.3 ± 3.2 MJ m⁻³ vs. 3.3 ± 1.0 MJ m⁻³). The stress–strain curves of ductile UPy-LBL copolymers (Fig. S8†) exhibited strain softening after yield, which could be attributed to the hydrogen bond dissociation upon higher stress.

Mechanical properties as a function of UPy-functionalization

As described before, neat PLA, UPy-PLA (F = 2), UPy-PLA (F = 1) and LBL (80-3.3-80, F = 0) exhibited brittle character, as the samples failed after minimal deformation (ε_TS < 15%, Fig. S7†). Upon addition of UPy into the chain ends (F = 0.2 to 0.4), the polymer exhibited brittle character with local whitening. However, once the functionality was increased above 0.4, the polymer began exhibiting more ductile behavior (sample exhibits neck deformation and ε_TS > 15%). In fact, UPy-LBL hemitelechelic copolymer (F = 1.0) was the toughest of the series with a 4.4-fold increase in toughness compared with PLA and same functionality UPy-PLA, F = 1.0 (Fig. 2). The stress–stain curves of this polymer (Fig. S9†) depicted ductile behavior with strain softening. After yielding, cold drawing with a constant engineering stress (∼25 MPa) was observed. The general whitening observed in the stress zone of these ductile materials (Fig. 3) has been linked to a high number of nanoscale fibrils formed by the crazing deformation mechanism, which is indicative of increased toughness (Fig. 5).³¹ Increased functionalization (F = 2) resulted in decreased toughness compared to the hemitelechelic polymer, but still tougher (∼2.8 fold) than neat PLA or UPy-PLA.³² On the other hand, UPy-functionalization does not seem to affect the Young’s modulus (E) of the materials, varying from 1.6–2.2, where the sample with higher UPy-functionalization, UPy-LBL (80-3.3-80)-2.0, exhibited the lowest modulus (1.6 ± 0.1 GPa).

Fig. 5 Representative tensile bars illustrate the PLA samples before and after tensile testing. Strain whitening and cold drawing of PLA are most evident in the UPy-LBL(80-3.3-80)-1.0 sample.

One method used to toughen glassy materials is inducing the occurrence of cavitation as a dissipation mechanism in the brittle material. Cavitation voids are highly dependent on the crosslinking density of the material, rubber particle size, and the yield stress of the matrix polymer.^33,34 The cavity size should be small enough to efficiently dissipate the stress and toughen the material, yet large enough to effectively decrease the crack propagation. Therefore, to understand how hydrogen bonding alters the deformation mechanism of the LBL triblock copolymers, craze morphologies were analyzed by SEM.

The increase in the number of crazes and craze fibrils perpendicular to the elongation axis at the highly stressed region on the UPy-functionalized polymer, compared with the LBL(80-3.3-80) polymer, could explain the overall whitening observed in this sample (Fig. 3(2a)–(c)).^35,36 These crazes and fibrils were possibly due to increased apparent entanglement density caused by hydrogen bonding compared with neat PLA. Hydrogen bonding can occur between the UPy moieties and PLA carbonyls in addition to UPy–UPy dimerization. Necking of the polymer could be caused by alterations in this entanglement network (i.e., hydrogen bonding) that allow the dissipation of energy through strain delocalization as the plastic is deformed. This dissipation of energy can favor the formation of crazes and fibrils instead of void growth that prevents premature crack propagation.^1,4,37,38 Craze formation is also favored at increased molar mass.³¹ This proved to be true for these materials as crazes were only observed at molar mass values over 160 kg mol⁻¹.³⁹

Aging studies

PLA and other polymer glasses have been reported to lose ductility as a result of physical aging. Understanding the phenomenon of aging is crucial to understanding the behavior of polymers during long-term applications or over prolonged storage.^28,40,41 The physical aging of polymers is a process where quenched amorphous materials in the glassy state relax toward equilibrium via structural relaxation. This process is responsible for the time-dependent changes in physical properties such as thermal conductivity and gas permeability.^40,42 During the relaxation process the polymer chains reorganize increasing their density and decreasing their enthalpy.^22,28,43,44

As reported, the physical aging of glasses is highly dependent on the polymer topology, entanglement density, and inter-chain free volume.^28,45 Therefore, aging effects can be mitigated by crosslinking or reinforcing the material. The addition of hydrogen bonding substituents could reduce segmental mobility and the rate of physical aging. Investigations into this approach have found that changes in polymer chain stiffness and the extent of hydrogen bonding likely plays key roles.^22,46

The physical aging of ductile UPy-LBL(80-3.3-80)-1.0 was first followed by DSC (Fig. S10†). The heating thermograms exhibited an increase in magnification and a shift to higher temperature for the T_g transition as a function of annealing time. Therefore, the unaged sample (the sample immediately after compression molding) showed a T_g of 55 °C, which increased to 59 °C after 360 h of annealing at 40 °C. This increase is in agreement with previous aging studies, where more energy is necessary to reach T_g to overcome structural rearrangements caused by aging.^28,44

The physical aging of UPy-LBL(80-3.3-80)-1.0 was then followed by uniaxial tensile testing (Fig. 4). The stress–strain curves showed that unaged polymers elongated about 210% beyond the original length with observed strain whitening and necking. There was a significant reduction in the fracture strain with increased aging time, and therefore the sample had an ultimate elongation of only 17% after five days of aging at 40 °C. These results are remarkable compared to amorphous poly(L-lactide), which becomes brittle after only 1.5 h of accelerated aging at the same temperature.²⁸

Conclusions

We have demonstrated that by functionalizing the triblock copolymer poly(D,L-lactide)-b-poly(butadiene)-b-poly(D,L-lactide) with 2-ureido-4[1H]-pyrimidinone (UPy), an improvement in mechanical deformation behavior can be achieved. UPy-LBL polymers with high PLA content (>96 wt%) increased the tensile toughness from 3.3 to 9.3 MJ m⁻³ compared with neat PLA. Tensile testing confirmed a correlation between the mechanical properties and the UPy-content, finding higher ductility for materials with over 0.6 UPy-functionalization. From functionalization studies, it was found that the hemitelechelic polymer was the toughest of the series with a tensile toughness 4.4 times greater than PLA. This change in ductility was demonstrated by SEM, which showed an increase in craze formation in the stressed region of the tensile bar. Hydrogen bonding was also found to delay the embrittlement time of aged samples. This is possibly the result of decreased segmental mobility, which reduces the polymer chain rearrangement involved in physical aging.

Acknowledgements

Funding for this work was provided by the Center for Sustainable Polymers at the University of Minnesota, a National Science Foundation supported Center for Chemical Innovation (CHE-1136607). Part of this work was carried out at the Institute of Technology Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network. SAXS data were acquired at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E. I. Dupont de Nemours and Co., The Dow Chemical Co., and the State of Illinois. Use of the APS was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. DE-AC02-06CH11357. The authors would like to thank Grayce Theryo for helpful discussions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra00150h

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