Rheology of Poly(Lactic Acid)

Abstract

The readers of this chapter are expected to come from a wide variety of backgrounds with varying degrees of expertise in polymer materials science. Accordingly, a detailed understanding of the rheological properties of polymers is, for many readers, an unknown subject. With these factors in mind, the first topic to be addressed is, What is rheology and why is it important?Simply stated, rheology is the study of the flow and deformation of matter. Rheology is a subfield of continuum mechanics that combines elements of both solid and fluid mechanics. Polymer rheology is a highly advanced science that reveals very important features of polymeric materials like PLA. In particular, the viscometry of dilute polymer solutions is an effective means of determining fundamental properties of polymer molecules; these include the molecular weight and the characteristic ratio (subjects to be discussed in detail in this chapter). More important, the melt rheology of polymeric materials reflects the relationship between molecular structure and dynamic properties. Of these properties, the viscosity and elasticity of the molten polymer are of primary importance in processing of plastics into useful articles. For example, both the shear thinning viscosity and the strain hardening extensional behavior of PLA are of great importance in plastic processing operations such as extrusion and fiber spinning. With this very cursory discussion of the nature and importance of rheology in mind, the attentive reader will ask, Is there anything particularly unique about the rheology of PLA?Indeed, the melt rheology of PLA is distinct from other commodity plastics in at least one important respect. Namely, the chemical stability of PLA can be adversely affected rather easily compared to most other polymeric molecules. At processing temperatures typically needed for semicrystalline PLA, namely, those above the melting temperature, both hydrolysis and pyrolysis can become important. Fortunately, commercially available PLAs are typically stabilized against thermal degradation. However, it is impossible to overstate the importance of drying PLA sufficiently prior to processing (or measuring its rheological properties!). Rheological measurements can provide direct evidence for the loss in molecular weight of improperly stabilized PLA in the molten state. Accordingly, in examining rheological data on PLAs, it is of particular importance to understand what, if any, chemical changes have taken place during the preparation and testing of the samples. In this regard, there is a clear link to dilute solution viscometry that can provide information about molecular weight in a simple test requiring only elementary equipment. As a final remark on the unique nature of PLA rheology, we can state that rheological measurements are particularly useful for assessing the effectiveness of both chemical stabilization against thermal degradation and of drying procedures meant to preclude hydrolytic degradation.This chapter is organized to provide a short yet detailed un...

Insight into industrial PLA aging process by complementary use of rheology, HPLC, and MALDI

“…G N 0 values estimated from the curves are as follows: 3.97 × 10 5 Pa for mixture PLA and 2.42 × 10 5 Pa for natural PLA. These results are in good agreement with previous values reported by Dorgan (5 × 10 5 Pa), Cooper‐White et al . (5.5 × 10 5 Pa), and Grijpma et al .…”

“…Tuminello et al . showed that MWD can be determined from linear viscoelastic melt properties, such as the shear storage modulus, G ′( ω ), or the stress relaxation modulus G ( t ), and from viscosity versus shear rate curves …”

Effect of Maleic‐Anhydride Grafting on the Physical and Mechanical Properties of Poly(L‐lactic acid)/Starch Blends

“…The flow rate was 1 mL · min −1 , runtime was 40 min, and temperature was 35 °C. The Mark‐Houwink constants K = 0.0131 (mL · g −1 ) and a = 0.759 for chloroform at 30 °C for dilute PLLA solution were used 33…”

Effect of MIL‐53 (Al) MOF particles on the chain mobility and crystallization of poly(L‐lactic acid)

“…To disregard the thermal history during processing or storage, the second heating cycle was used to determine the glass transition temperature ( T g ), cold crystallization onset ( T co ), melting temperature ( T m ), and to calculate the enthalpy of cold crystallization ( ΔH c ) and fusion ( ΔH m ). The percent crystallinity X c (%) was estimated using the following equation: where Δ H m is enthalpy of fusion; Δ H c is the enthalpy of cold crystallization; Δ is enthalpy of fusion of pure crystalline PLA (Δ = 93.1 J/g) and x represents the mass fraction of the MOF in the films. Universal Analysis software version 2000 (TA Instruments, DE) was used to analyze the data.…”