Influence of processing parameters on the molecular weight and mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Abstract

The influence of film blowing and melt extrusion under different processing conditions on the mechanical properties and molecular weight of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) have been characterized. Natural polymers are generally more sensitive to the high temperatures used in processing than synthetic ones. Size exclusion chromatography (SEC) showed that the weight average molecular weight M ̄ w decreased with increasing processing temperature and decreasing screw speed. A 50% difference in M ̄ w could be achieved. The modulus of elasticity ( E) exhibited only minor changes with processing conditions and was, consequently, not affected by the M ̄ w . The elongation at yield ( ϵ y) and break ( ϵ b) decreased significantly with increasing processing temperature with a maximum at a screw speed of approximately 25 rpm. The stress at yield ( σ y) decreased with increasing processing temperature. The stress at break ( σ b) showed a larger variation with processing conditions with values between 21 and 31 MPa. The blown film exhibited values of M ̄ w , and σ b, comparable with those of the extruded samples. The film showed no yield before break and was rather brittle. Both ϵ y and ϵ b of the extruded samples decreased approximately linearly with decreasing M ̄ w , and a breaking point in the tensile strength was evident at a value of about 150 000 g/mol below which the mechanical properties decreased rapidly. In addition, σ y and σ b decreased with decreasing M ̄ w . The mechanical properties are optimized at a processing temperature of 160–165 °C and a screw speed at about 25 rpm, although it is suggested that these conditions are not optimal if the desire is to achieve rapid embrittlement and disintegration of the polymer.