Characterization of Bio-Derived Polymer Under Controlled Humidity

Glass Transition Dependence on Humidity

Figure 2 shows oscillation temperature ramp data, where tan delta is plotted as a function of temperature for various humidity setpoints. Moisture acts as a plasticizer to PLA, effectively lowering the strength of the molecular interactions of the polymer matrix, thus lowering the glass transition. [4,5] The peak points of the tan delta represent one method of determining the glass transition temperature. As humidity increases, the effective glass transition temperature decreases. Between a relative humidity of 5-80%, the glass transition temperature varies over 10 °C (62-73 °C). This is a major consideration for not only processing various grades of PLA, but also the softening point (upper thermal limit, heat deflection temperature) which has implications for the final product. This effect is well known for Nylon, among other polymers, but is also of critical importance for bio-derived and/or biodegradable polymers.[4] Polymer processers can use this information to observe how humidity may be introducing variability to their process, or to condition a polymer to a humidity level to take advantage of the plasticizing effect of the moisture.

Viscoelastic Response to Moisture at Different Temperatures

Frequency sweeps provide complex viscoelastic data such as storage modulus, loss modulus, tan delta as a function of the deformation frequency, effectively measuring the mechanical properties, or the extent to which a material responds in a liquid or elastic modality and related molecular relaxation processes as a function of timescale or frequency. The viscoelastic data as a function of frequency for the PLA films has been characterized at a variety of humidity settings (5, 30, 55, 80% RH), and for three overall temperatures all below the glass transition of the PLA (25, 37.5, and 50 °C relative to a glass transition temperature range of 62-73 °C).

Figure 3 features an overlay of frequency sweep data at each humidity setpoint, at a testing temperature of 25 °C. Storage modulus and tan delta are both plotted as a function of frequency. As a result of increased humidity, storage modulus decreases slightly and tan delta increases, which both demonstrate the softening of the PLA. It is worth noting, however, that the changes are very small in magnitude at this temperature.

This contrasts with the results at 37.5 °C, shown in Figure 4, where again storage modulus and tan delta are plotted as a function of frequency for the various humidity setpoints. While changes to the storage modulus are still marginal with increased humidity, the changes to tan delta are much more substantial at this temperature. The increase in the tan delta with humidity indicates that the PLA is becoming less elastic and more viscous in terms of mechanical response.

Finally, in Figure 5 storage modulus and tan delta are plotted as a function of frequency for the various humidity setpoints, when the PLA sample is held at a temperature of 50 °C. At this temperature, the effect of humidity on the mechanical response is considerably larger than at the previous temperatures. At a frequency of 0.1 Hz, the change in storage modulus and tan delta are over a decade in magnitude, when considering humidity setpoints of 5% and 80%. Additionally, the complex mechanical response also has a much stronger frequency dependence, which wasn’t observed at the other temperatures.

This data illustrates that even below the glass transition temperature, the viscoelastic properties of semicrystalline PLA polymer still show sensitive response to the environmental temperature and humidity, where the polymer exhibits different mechanical properties than expected. These measurements allow a producer or manufacturer to define limits for the use of the material, which are dependent on the environment.

Hygroscopic Expansion

Analogous to the changes in mechanical response due to a combination of temperature and moisture as seen in the last section, the expansion or contraction of a material can also be characterized in response to these factors. From an oscillation time sweep step in procedure, the length change of the sample can be observed as a function of time.

Figure 6 features oscillation time sweep data, where sample length is plotted as a function of time for temperatures of 25, 37.5, and 50 °C respectively. The curves in each plot are representative of sample length after held at the setpoint environmental temperature and the various humidity setpoints. From the starting point of each curve, the humidity is ramped in increments every 300 minutes (see inset), the ramping period can be observed through small gaps in the curves.

At 25 °C, it can be observed that regardless of the humidity setpoint, the magnitude of sample expansion over the course of the observation window is negligible. When the temperature is increased to 37.5 °C, the sample undergoes a bit more elongation but the overall percent change is still quite small, expanding approximately 1% in length. This demonstrates that at these temperatures and humidity setpoints, the changes in the sample dimensions in still very small.

When held at 50 °C, the sample undergoes significant expansion at elevated humidity setpoints. At a humidity setpoint of 55%, around 800 minutes into the experiment, the sample elongates over 20% from the start to the end of the oscillation time step. This effect is accelerated at higher humidity setpoints, until the sample is saturated with moisture and the length starts to reach a plateau. Between 37.5 °C and 50 °C and above 30% relative humidity, the sample has major dimensional expansion compared to the lower setpoints. This sets an upper limit on the environmental conditions that this grade of PLA can be used in before the product geometry expands.