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How can a plasticizer lower the glass transition temperature of PVC?

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plastic soft-drink bottles — plastic Plastic soft-drink bottles are commonly made of polyethylene terephthalate (PET).

Plasticizers lower the glass transition temperature ( $T_{g}$ ) of polyvinyl chloride (PVC) by acting as internal lubricants that increase molecular motion within the polymer matrix.12

In its pure, unplasticized state, PVC is a rigid material with a high $T_{g}$ (typically 85 to 90 °C), as its polymer chains are essentially frozen in a glassy state at lower temperatures. When nonvolatile liquids—such as phthalates—are mixed into the PVC, they disperse between the polymer chains. These additives increase the free volume between the chains and facilitate greater molecular movement, which allows the material to transition from a rigid, glassy state to a flexible, rubbery state at significantly lower temperatures.2341

By adjusting the concentration of these plasticizers, manufacturers can tailor the $T_{g}$ of the final product to suit specific applications, such as lowering it to approximately -10 °C for flexible items like garden hoses.2