▁transition ▁temperature ▁and ▁below ▁its ▁mel ting ▁point , ▁the ▁physical ▁properties ▁of ▁a ▁therm op last ic ▁change ▁dr ast ically ▁without ▁an ▁associated ▁phase ▁change . ▁Some ▁therm op last ics ▁do ▁not ▁fully ▁cry st all ize ▁below ▁the ▁glass ▁transition ▁temperature , ▁retain ing ▁some ▁or ▁all ▁of ▁their ▁am orph ous ▁characteristics . ▁Am orph ous ▁and ▁semi - am orph ous ▁pl ast ics ▁are ▁used ▁when ▁high ▁optical ▁clar ity ▁is ▁necessary , ▁as ▁light ▁is ▁scattered ▁strongly ▁by ▁cry st all ites ▁larger ▁than ▁its ▁w avel ength . ▁Am orph ous ▁and ▁semi - am orph ous ▁pl ast ics ▁are ▁less ▁resist ant ▁to ▁chemical ▁attack ▁and ▁environmental ▁stress ▁crack ing ▁because ▁they ▁lack ▁a ▁cry st all ine ▁structure . ▁ ▁Brit t len ess ▁can ▁be ▁decre ased ▁with ▁the ▁addition ▁of ▁pl astic izers , ▁which ▁increases ▁the ▁mob ility ▁of ▁am orph ous ▁chain ▁segments ▁to ▁effectively ▁lower ▁the ▁glass ▁transition ▁temperature . ▁Mod ification ▁of ▁the ▁polym er ▁through ▁cop ol ym er ization ▁or ▁through ▁the ▁addition ▁of ▁non - re active ▁side ▁ch ains ▁to ▁mon om ers ▁before ▁polym er ization ▁can ▁also ▁lower ▁it . ▁Before ▁these ▁techniques ▁were ▁employed , ▁pl astic ▁autom obile ▁parts ▁would ▁often ▁crack ▁when ▁exposed ▁to ▁cold ▁temper atures . ▁These ▁are ▁linear ▁or ▁slightly ▁branch ed ▁long ▁chain ▁mole cules ▁capable ▁of ▁repeatedly ▁soft ening ▁on ▁he ating ▁and ▁hard ening ▁on ▁cool ing . ▁ ▁Ac