Chain extended poly(3-hydroxybutyrate) with improved rheological properties and thermal stability, through reactive modification in the melt state

Abstract

Branching and crosslinking of polyhydroxybutyrate (PHB) is accomplished for the first time through reactive modification in the melt state, using dicumyl peroxide (DCP) as the free-radical initiator and a tri-functional coagent, triallyl trimesate (TAM). The dynamic oscillatory rheological properties, as well as extensional viscosities provide strong evidence of branching and crosslinking, depending on the amounts of DCP and TAM. In addition to the significant increases in viscosity due to branching/crosslinking, coagent modified samples exhibit a significant increase in the crystallization temperature, indicative of a nucleating effect, accompanied by a decrease of crystal spherulite sizes. In addition, the solid-state storage modulus in DMTA experiments is 65% higher in formulations that contain high gel contents, whereas the glass transition temperature of the cured formulations is slightly lower. Coagent modified PHB exhibit a substantial improvement in the thermal stability, both in isothermal dynamic oscillatory temperature sweeps, as well as non-isothermal TGA experiments.

Thermal and rheological properties of modified polyhydroxybutyrate (PHB)

“…A similar behavior of coagent‐modified PHB is described in Ref. . It is proposed that this can be ascribed to a highly long chain branched structure with partially crosslinking and pronounced elastic properties.…”

“…From literature it is known that the spherulite size of PHB decreases enormous by branching/partial crosslinking resulting in a pronounced nucleation and thus in an enhanced crystallization temperature. An explanation might be that the mobility of the PHB chain segments which are near the chemical linkage of the modifier is limited.…”

“…In this work TBPB seemed to be a suitable peroxide because complete decomposition of the peroxide as well as sufficient reaction time during extrusion could be ensured. But as reported at polymers like polypropylene and PHB processing in the presence of free‐radicals cause often severe chain scission. The reaction has then to be assisted by a bi‐ or multi‐functional coagent/modifier with unsaturated sites which can react with the induced free radical on the polymer backbone.…”

“…Another possibility to retard thermal degradation of PHB is the grafting of various unsaturated monomers, like acrylics as methyl methacrylate, 2‐hydroxyethyl methacrylate, and acrylic acid or maleic anhydride and styrene, respectively . Also the partial branching and crosslinking by an organic peroxide like dicumyl peroxide and a multifunctional coagent, such as triallyl trimesate as well as by the epoxy‐functionalized chain extender Joncryl are routes to improve thermal stability by an increase in activation energy of thermal decomposition . But most of these mentioned routes were performed in solution, in discontinuous batch processes with the help of a laboratory kneader (e.g., from Thermo Scientific Inc. or Brabender GmbH & Co KG) or in a mini‐extruder where in a separate step the PHB have to be coated with the adequate reactive component before feeding into the extruder.…”

Advances in drug delivery systems based on synthetic poly(hydroxybutyrate) (co)polymers

“…in the melt state using a dicumyl peroxide/triallyl trimesate (DCP/TAM) afforded high degrees of PHB branching and/or crosslinking depending on the amounts of DCP and TAM [142]. Dynamic oscillatory rheological properties of TAM(PHB-b-DCP-b-PHB)3, as well as extensional viscosities showed that with added amounts of DCP (0.2 and 0.3wt%) and TAM (up to 1wt%), a gel fraction of up to 65wt% was obtained and the viscosity of PHB increased by about two orders of magnitude from unmodified PHB to DCP/TAM-modified PHB, thus suggesting that TAM was a very effective branching/crosslinking agent for PHB.…”