Tran Hai Ninh - Polymer Center- HUTS *Chng 7: Tnh cht c hc ca polyme(Mechanical Properties of Polymers) Khi lng ring thp cng vi cc tnh cht c trng ca polyme lm cho polyme c th thay th kim loi v gm trong nhiu lnh vc ng dng. bn c hc ca vt liu polyme lun l yu t quan trng n nh c hc c xem xt bn cnh cc tnh cht khc nh tnh cht in, quang hoc nhit

Tran Hai Ninh - Polymer Center- HUTS

7.1. Cc php o tnh cht c hca. Php o ng sut-bin dng Mu b bin dng (ko, nn, un) vi tc khng i, ng sut v bin dng c ghi li lin tc.

Hnh dng v kch thc mu dng cho php th koThit b o tnh cht c hc (ko, nn, un)

ng sut (k thut):F : lc tc dngAo : din tch mt ct ngang ban u ca mu koBin dng (k thut):Lo : chiu di ban u ca muL : chiu di mu thi im ong cong ng sut bin dng

7.1. Cc php o tnh cht c hcb. Php o do (creep) Mu chu tc dng lc khng i, bin dng c ghi li theo thi gian (bin dng ph thuc thi gian) Php th do c thc hin khi mu b ko (a s), nn, un, xon, v trt.

Creep Testing Machine

7.1. Cc php o tnh cht c hcc. Php o hi phc ng sut Mu ngay lp tc b ko n 1 i lng bin dng no , ng sut duy tr bin dng gim dn theo thi gian.

PMMAM un hi phc

7.1. Cc php o tnh cht c hcd. Php th va p (Impact test) Ti trng tc dng ln mu vi vn tc rt ln (2m/s) Php th va p a ra nhng thng tin hu ch la chn vt liu polyme cho cc ng dng c bit.

Impact testerMu o php th va pTrcSau khi mu b ph hy

Nhng polyme thng dng nh PS c bn va p thp trong khi nha k thut nh polyamit, polycacbonat, v polyoxymetylen v mt s nha nhit do thng dng khc nh PE, PP v polytetrafloetylen c bn va p cao hn.

7.2. Cc quy lut bin dng c bn ca polyme trng thi rna. Bin dng n hi (elastic deformation) Di tc dng ca ngoi lc, bin dng xy ra do cc nguyn t hoc phn t di xa nhau mt khong cch no (gc ha tr b bin dng, lin kt ha hc b ko cng), nhng bin dng ny mt i nhanh khi gii phng lc tc dng. Bin dng n hi l bin dng thun nghch.Bin dng n hi

nh lun Hc (Hookes law) Mun n hi: E l hng s c trng cho tng vt liu, n biu din kh nng chng li bin dng ca vt liu ngha vt l ca E ?

b. Bin dng do (plastic deformation) Bin dng do ca vt rn l bin dng khng thun nghch Lm thay i hon ton trt t sp xp ban u ca nhm nguyn t hoc phn t nhng khng lm thay i khong cch gia cc phn t (th tch ring ca h thng khng i).S dch chuyn nh vy lm thay i hnh dng vt th nhng khng lm thay i ni nng. Bin dng do pht trin nhanh hn lc tc dng.Bin dng do

H s puason (Poissons ratio) = 0.5 i vi bin dng do

c. Bin dng mm cao (rubbery deformation) Bin dng mm cao ging vi bin dng do khi khng lm thay i khong cch gia cc phn t h s = 0.5, trong khi bin dng n hi c < 0.5. Bin dng mm cao ging bin dng n hi tnh thun nghch. Bin dng mm cao cn c thi gian pht trin, trong khi bin dng n hi xy ra tc thi. Bin dng mm cao ch xut hin trong mt khong nhit xc nh? Nguyn nhn c bin dng mm cao l do linh ng, mm do v kh nng thay i hnh thi sp xp ca i phn t.

ng x ng sut - bin dng ca xenlulo axetat cc nhit khc nhau

7.3. ng cong ng sut bin dng ca polyme trng thi rn Mt s thng s c bn c trng cho tnh cht c hc ca vt liu polyme ly ra t ng cong ng sut bin dng

cng (Stiffness) kh nng ca vt liu chu ng sut m khng thay i kch thc. Gi tr ca modun n hi c trng cho kh nng ny. Tnh n hi (Elasticity) kh nng chu bin dng thun nghch hoc chu ng sut m khng c bin dng d. N c xc nh bng gii hn n hi (theo quan im thc nghim), gii hn chy (yield point, proportional limit) dai (Resilience) kh nng hp th nng lng m khng c bin dng d (permanent deformation). L phn din tch ca vng bin dng n hi ca ng cong ng sut bin dng. bn (Strength) kh nng vt liu chu c n khi b ph hy. c c trng bng bn ko hoc ng sut ti mu b ph hy hon ton (rupture). bn dai (Toughness) kh nng hp th nng lng bao gm c phn bin dng do n khi mu khng b ph hy. c o bng din tch pha di ton b ng cong ng sut bin dng.

Mt s ng cong ng sut bin dng c trng cho vt liu polyme

Thay i hnh dng ca mu trong php th ko nhit thng ca mt s polymeHin tng c eo

Khi bin dng nh, c polyme v nh hnh v polyme tinh th u th hin bin dng n hi. i vi polyme cng v dn (v d PS) khng c hoc rt kh quan st c gii hn chy, Nu tng ng sut th mu b ph hu dn (ng 1). i vi polyme dai (nha nhit do k thut nh PA, HIPS) th trn ng cong ng sut - bin dng c im cc i (cao hn gii hn chy mt cht) v sau ng sut gim n gi tr cc tiu. Qu trnh hnh thnh v pht trin c eo (necking phenomena) l nguyn nhn ca qu trnh chy lnh.

7.4. Cc yu t nh hng n tnh cht c hc ca polymea. nh hng ca khi lng phn tnh hng ca khi lng phn t n ng cong mun trt - nhit .Tg - nhit ho thu tinhTfl - nhit chy nhtKLPT thp KLPT trung bnhKLPT caoNu Tg > nhit phng ?Nu Tg < nhit phng ?

b. Anh hng ca mt lin kt ngangnh hng ca lin kt ngang n ng cong mun trt - nhit ca cao suKhng c lin kt ngangMcMcMt lin kt ngang tng lnTrng thi thu tinhTrng thi mm cao

c. nh hng ca hm lng tinh thPha tinh th c moun cao hn pha v nh hnh, c xem nh pha gia cng trong nn v nh hnhHm lng tinh th (%) tng lnTg

d. nh hng ca cht ho doCht ho do l hp cht thp phn t, thng l cht lng c nhit si cao.Cht ho do c tc dng lm tng kh nng chy ca polyme bng cch lm gim nhit ho thu tinh.M un, gii hn chy v bn ko ca vt liu polyme u b gim khi s dng cht ho do

e. nh hng ca phn ccTg (PVC) = 90oCTg (PP) = -15oCnh hng ca (a) moun n hi, G, v (b) h s tn hao, tg vo nhit PVCPP

f. nh hng ca nhit So vi ceramic v kim loi, tnh cht c hc ca polyme ph thuc rt ln vo nhit .Mun ca polyme gim khi nhit tng. Mc gim ln nht nhng nhit chuyn pha nh Tg v Tmng x ng sut - bin dng ca xenlulo axetat cc nhit khc nhau

g. nh hng ca vn tc bin dngKhi vn tc bin dng tng, do dai ca polyme gim trong khi moun v gii hn chy hoc bn ko tng ln.Polyme gin th hiu ng ny tng i nh, tuy nhin polyme dai v n hi th hiu ng ny nh hng ng k.

g. nh hng ca p sutp sut nh hng rt ln n tnh cht c hc ca polymePPMoun tng v gii hn chy tng khi p sut tng (i vi tt c polyme !) bn ko ca polyme dai tng, cn ca polyme gin li gim khi p sut tng.

7.5. c trng ph hy polymePh hy dn (Brittle fracture) Ph hy dn l dng ph hy m trc ch c bin dng n hi thun nghch xy raPh hy do (Ductile fracture) Ph hy do l dng ph hy m trc ch c xy ra nhng bin dng gy nn do cc phn ring bit thuc cu trc vt liu sp xp li (chy do)

A rt do.B do C - Dn

Cc hin tng hi phc ca polyme Hin tng hi phc l hin tng polyme trng thi cn bng ny sang trng thi cn bng khc di tc dng ca ngoi lc (in, t, c hc) Qu trnh hi phc l qu trnh bin i theo thi gian ca polyme t trng thi khng cn bng n trng thi cn bng. Thi gian hi phc l thi gian cn thit thit lp cn bng mi