Article pubs.acs.org/JPCB
Analysis of Structural Rearrangements of Poly(lactic acid) in the Presence of Water Omkar Vyavahare, David Ng, and Shaw Ling Hsu* Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States ABSTRACT: The effects of hydration on two types of structural rearrangements, physical aging and crystallization, of poly(lactic acid) (PLA) have been characterized. Although the water absorbed is extremely small, on the order of 0.5% by weight, we found a significant increase in both the magnitude and kinetics of enthalpic relaxation near the glass transition temperature (Tg). The kinetics of aging is at least 2 times the speed as compared to dry samples. In addition, we also found that the crystallization kinetics can be increased significantly when PLA is hydrated. The initial rate of crystallization at 80 °C is extremely fast at 7 J/g/min for the hydrated state as compared to 2 J/g/min under dry conditions. However, the ultimate degree of crystallinity achieved is not different for the two types of samples. Both of these structural rearrangements in the hydrated state can be explained by an increase in segmental mobility. The fact that water has such a strong effect on PLA structure is attributed to the strong intermolecular interactions present and their changes in the hydrated state. Spectroscopic features associated with bound water or free water were found for PLA with different hydration levels. The increase in the segmental mobility was directly correlated to the presence of free liquid water disrupting the intermolecular interactions in PLA.
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INTRODUCTION There has been a tremendous interest in poly(lactic acid) (PLA) owing to its attractive properties such as biodegradability, biocompatibility, and processability.1−5 PLA can be derived from renewable resources, and it degrades into nontoxic lactic acid which is biocompatible. Thus, PLA finds applications both as commodity plastics such as fibers, films, and adhesives and in biomedical fields such as coronary stents, bone fixation devices, sutures, and drug delivery vehicles.6,7 Water absorption of polymers, including PLA, is an important topic from both scientific and technological perspectives. Interaction of water with the polymer depends on the polymer structure, morphology, and temperature. Depending on the extent of interaction, absorbed water can exist in the polymer matrix in different states such as “bound” water having specific interactions with the polymer and “free” water with no interactions and having bulk water-like characteristics.8−10 Initially, analyzing water effects on PLA does not seem to be promising, since the water absorption in PLA does not exceed 1% by weight. In fact, water absorbed is usually reported in the 0.5% range.11−14 This amount translates into approximately one water molecule for every 30 PLA monomer units. Even if we assume that the water can only have sufficient permeability into the amorphous region, for a 50% crystalline sample, which would then suggest a molar concentration of one water molecule for every 15 PLA monomer units. However, a number of physical characteristics of PLA imply the effect of water may be more significant than the small numerical value would imply. Previous studies have clearly established that the specific intermolecular electrostatic interaction dominates the chain packing of PLA.15−17 Water can then preferentially perturb these intermolecular inter© 2014 American Chemical Society
actions, causing possible structural transformations. Therefore, the magnitude and kinetics of water-induced structural transformations need to be more fully understood. There have been significant efforts carried out over the last 40 years to assess the effects of water on polymers. Water can affect chain dynamics in two very different but somewhat opposite ways. Water can act as a plasticizer for polymeric systems, enhancing the segmental mobility and disrupting the intermolecular interactions.18,19 Extensive studies of water absorbed into various nylons are some specific examples.20,21 The segmental mobility increase (plasticization) as a result of the increase in the specific volume has been used to interpret the decrease in the glass transition temperature and other physical properties. In contrast, in certain biopolymers such as wheat gluten and starch based food systems, it has been reported that water increases the modulus, making them brittle and stiffer (antiplasticization).22,23 These changes in properties resemble the effects of antiplasticizers on polymers such as polystyrene or polycarbonate.24−26 Such an increase in the modulus has been attributed to a decrease in free volume and an increase in the polymer−diluent interaction, leading to steric hindrance, and therefore a decrease in segmental mobility.24−26 It has been reported that water has a plasticizing effect on PLA, reducing its Tg and physical properties.12,13 In the absence of water, the morphological features of PLA and their changes are already quite complex. The segmental dynamics and chain conformational distribution can be influenced by chain configuration and experimental conditions.1,27,28 The formation Received: June 27, 2013 Revised: March 25, 2014 Published: March 27, 2014 4185
dx.doi.org/10.1021/jp500219j | J. Phys. Chem. B 2014, 118, 4185−4193
The Journal of Physical Chemistry B
Article
aluminum hermetic pans and run at a heating/cooling rate of 10 °C/min with nitrogen gas purge set at 50 mL/min. The results were analyzed using TA Instruments Universal Analysis software. The glass transition temperature was taken at the midpoint of the temperature range over which glass transition takes place between onset and end set of the inflectional tangent. Enthalpy loss (δH) was determined from the endothermic peak area difference between the annealed sample and the pristine sample. For the samples crystallized during the DSC measurements, the degree of crystallinity (Xc) was measured by subtracting the enthalpy of crystallization (ΔHc) obtained by integrating the crystallization peak as well as the small exothermic peak just before the melting peak (ΔHα′−α) from the enthalpy of fusion (ΔHf). The degree of crystallinity was obtained by eq 1 using the theoretical value of enthalpy of fusion (ΔHf°) of 93 J/g for the 100% crystalline PLA33 as follows:34,35
of the condensed state can also vary depending on the processing conditions of time and temperature. In addition, there is no question that PLA can “age” and “cold crystallize”, leading to very different physical properties. Such effects have been explained in terms of densification,29 conformational rearrangements,30 formation of local order due to cohesional entanglements,31,32 or enhanced mesomorphic structure.2 These changes have been modeled as a function of chain configuration and temperature. The presence of strong intermolecular interactions plays a strong role in the explanations provided. As compared to other polymers, the effects of water on PLA are even more intriguing. In many applications, PLA is bound to interact with water, especially in the biomedical field. Thus, it is critical to understand how the interaction between PLA and water affects the microstructure and the concurrent changes in mechanical properties. Because of the specific interactions that exist, it is our hypothesis that water has the potential to significantly affect the PLA structure, stability of the individual phases, and macroscopic properties. However, to our knowledge, there has not been any comprehensive study on these subjects. Thus, it is critical to determine the effect of water on the intermolecular interactions (hydrogen bonding, dipole interactions, or electrostatic interactions) of PLA which could alter the physical properties. In the present study, we are interested in determining how water influences the kinetics of the structural rearrangement of PLA. It is important to characterize the mechanism by which hydration of PLA leads to densification, additional crystallization, or change in the crystalline phase and the interrelationship of these physical phenomena.
%Xc = (ΔHf − ΔHc − ΔHα ′− α) × 100/ΔHf°
(1)
Wide Angle X-ray Scattering (WAXS). WAXS scattering was conducted on an evacuated Rigaku Molecular Metrology instrument using the Cu Kα spectral line (λ = 0.1542 nm). A Fujifilm CR HR-V image plate at a distance of L = 139 mm was used to record the scattering. L was calibrated using q = 15.197 nm−1, the (110) peak from a tricosane standard. Scattering patterns from WAXS were circularly integrated to give scattering profiles with intensity in arbitrary units versus 2θ in degrees. The size of the crystallites was determined from the full width at half-maximum (fwhm) of the crystalline peaks using the Scherrer equation expressed as follows (eq 2):
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Bhkl (2θ ) =
EXPERIMENTAL SECTION Materials and Preparation. PLA with a D-isomer content of
