Aging of poly(lactide)/poly(ethylene glycol) blends. Part 2. Poly(lactide) with high stereoregularity

doi:10.1016/S0032-3861(03)00615-3

Polymer

Volume 44, Issue 19, September 2003, Pages 5711-5720

https://doi.org/10.1016/S0032-3861(03)00615-3 Get rights and content

Abstract

Blending poly(ethylene glycol) (PEG) with poly(lactide) (PLA) decreases the T_g and improves the mechanical properties. The blends have lower modulus and increased fracture strain compared to PLA. However, the blends become increasingly rigid over time at ambient conditions. Previously, it was demonstrated that a PLA of lower stereoregularity was miscible with up to 30 wt% PEG. Aging was due to slow crystallization of PEG from the homogeneous amorphous blend. Crystallization of PEG depleted the amorphous phase of PEG and gradually increased the T_g until aging essentially ceased when T_g of the amorphous phase reached the aging temperature. In the present study, this aging mechanism was tested with a crystallizable PLA of higher stereoregularity. Changes in thermal transitions, solid state structure, and mechanical properties were examined over time. Blends with up to 20 wt% PEG were miscible. Blends with 30 wt% PEG could be quenched from the melt to the homogenous amorphous glass. However, this composition phase separated at ambient temperature with little or no crystallization. Changes in mechanical properties during phase separation reflected increasing rigidity of the continuous PLA-rich phase as it became richer in PLA. Construction of a phase diagram for blends of higher stereoregular PLA with PEG was attempted.

Introduction

Poly(lactide) (PLA) is attracting increased attention for applications that require a biodegradable plastic because in a suitable disposal site it will degrade to natural products [1], [2], [3], [4], [5]. It also has good physical properties such as high strength, thermoplasticity, and fabricability. However, low deformation at break and high modulus have limited applications of PLA in the packaging industry. Attempts to improve the mechanical properties have focused on biocompatible plasticizers [6], [7], [8]. Poly(ethylene glycol) (PEG), the conventional name for low molecular weight (<20,000) poly(ethylene oxide) (PEO), improves elongation at break and softness of PLA [7], [8], [9], [10], [11]. At ambient temperature, the desired mechanical properties are achieved by blending PLA with 30 wt% PEG. However, there is evidence that the blend is not stable and the attractive mechanical properties are lost over time [11].

Lactic acid exists as two enantiomeric forms, the d(+) configuration and the naturally occurring l(−) configuration. These produce the corresponding enantiomeric polymers by conservation of the chiral center. Generally, commercial PLA grades are copolymers of l-lactide and d-lactide. The optical purity, defined as |l%−d%|, strongly affects the properties. Optically pure PLA is isotactic and highly crystalline. Decreasing the optical purity reduces the degree of stereoregularity and crystallinity. Poly(l-lactide) with more than 15 mol% d-lactide is amorphous [12].

Many studies indicate that tacticity of a constituent influences miscibility of polymer blends. Most studies focus on blends of poly(methyl methacrylate) (PMMA) because of its availability in both syndiotactic and isotactic forms. In blends with PEO and PEG the composition range of miscibility is larger for syndiotactic PMMA and for atactic PMMA (which contains long syndiotactic sequences) than for isotactic PMMA [13], [14]. Similarly, syndiotactic and atactic PMMA are miscible with poly(vinyl chloride) (PVC) over a larger composition range than is isotactic PMMA [15], [16]. It is suggested that these differences arise from the free-volume contribution to the energy of mixing.

Stereoregularity of PLA may similarly affect miscibility with PEG and other plasticizers. The phase condition has important implications for aging. However, previous investigations have not addressed this possibility. Blends of low stereoregular PLA with up to 30 wt% PEG are miscible at ambient temperature. Changes in mechanical properties over time are due to slow crystallization of PEG from the homogeneous blend. Depletion of PEG increases the glass transition temperature (T_g) of the amorphous phase until aging essentially ceases when T_g reaches the aging temperature [11]. The present study addresses the effect of stereoregularity on miscibility and aging of PLA/PEG blends. Results obtained with blends of a high stereoregular PLA are compared with findings in the previous study of low stereoregular PLA.

Section snippets

Materials

The study utilized two poly(lactic acid) resins, one with higher stereoregularity (PLA1) and one with lower stereoregularity (PLA2). The specific optical rotation, [α]_D²⁵, of PLA1 was −141 as measured in chloroform at a concentration of 1 g dl⁻¹ and 25 °C (Autopol III Polarimeter). From [α]_D²⁵, the PLA was determined to have a d-lactide content of 5% by assuming [α]_D²⁵ of poly(l-lactic acid) and poly(d-lactic acid) to be −156 and 156, respectively [12]. After annealing at 100 °C for 1000 min,

Unaged PLA/PEG blends

Thermograms of quenched PLA1 and PLA1/PEG blends are shown in Fig. 1. Without PEG, PLA1 was amorphous after quenching with T_g of about 60 °C. Quenched PLA1 did not cold-crystallize upon heating. Quenched PLA1/PEG blends were also amorphous and exhibited a single glass transition in thermograms. Increasing PEG content of the blend caused the T_g to decrease through ambient temperature to 12 °C for the blend with 30 wt% PEG. Upon heating the blends at 10 °C min⁻¹ the PEG did not crystallize,

Conclusions

The effect of stereoregularity of PLA in blends with PEG was studied. Although some of the blend constituents were crystallizable, the blends could be quenched from the melt to the homogeneous amorphous glass. Blends with up to 30 wt% PEG exhibited a single T_g that depended on composition in accordance with the Fox relationship. Addition of 30 wt% PEG put T_g below ambient temperature and thereby decreased the modulus and increased the ductility of this relatively rigid, brittle thermoplastic.

Acknowledgements

The generous financial and technical support of the Kimberly-Clark Corporation is gratefully acknowledged.

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Aging of poly(lactide)/poly(ethylene glycol) blends. Part 2. Poly(lactide) with high stereoregularity

Abstract

Introduction

Section snippets

Materials

Unaged PLA/PEG blends

Conclusions

Acknowledgements

Polymer

Polymer

Polymer

Polymer

Polymer

Polymer

Macromol Symp

Polym Engng Sci

Polym Engng Sci

ANTEC

Macromol Symp