Lecture 7 Notes (BME 296)

Lecture Overview

• Lecture Title: BME 296

• Topics Covered:

  • Review Concepts of Crystallinity

  • Transition Temperatures (Tm, Tc, Tg)

  • Effects on Metals, Ceramics, and Polymers

  • Presence of Crystalline and Amorphous Phases in Polymers

Review and Today

Key Questions to Explore:

• What is Crystallinity?

• What Affects Crystallinity?

• Tacticity:

  • Isotactic > Syndiotactic > Atactic

• Continuing Topics:

  • Transition Temperatures: Tm (Melting), Tc (Crystallization), Tg (Glass)

  • Effects of Temperature on Various Materials

Factors Determining Crystallinity

• Hierarchy of Tacticity:

  • Isotactic has highest crystallinity

  • Syndiotactic is intermediate

  • Atactic has lowest crystallinity

• Factors Influencing Crystallinity:

  • Linear vs. Branched Structures

  • Pendant Group Size:

    • Smaller side groups favor crystallinity over larger ones

Crystallinity in Polymeric Biomaterials

• Atactic Polymers:

  • More difficult to crystallize due to random side group placement

• Copolymers:

  • Regular arrangements (like block copolymers) promote better crystallinity

  • Alternating configurations increase crystalline regions' likelihood

Overview of Crystallinity in Polymeric Biomaterials

• Key Influences on Percent Crystallinity:

  • Side Groups: Bulky groups reduce crystallinity

  • Chain Branching: More branching leads to reduced crystallinity

  • Tacticity: Isotactic promotes highest crystallinity

  • Molecular Weight: Generally increased weight reduces crystallinity due to entanglement

  • Mechanical Properties Correlation: Higher molecular weight restricts chain mobility, limiting orderly packing

Effects of Crystallinity on Properties

• Crystallinity alters:

  • Mechanical Properties

  • Physical Properties

  • Cell Interactions

• Reference: Kołbuk et al. study on crystallinity effects on gene expression

Problem Regarding Copolymers

• Copolymers Given:

  • (a) -A-B-A-B-A-B-A-B-

  • (b) -A-B-B-B-A-A-B-A-B-

  • Analysis:

    • Chain (a) is alternating copolymer

    • Chain (b) is random copolymer

    • Chain (a) has higher probability of forming crystalline regions due to regularity

Temperature Effects on Polymers

• Heat increases molecular kinetic energy leading to motion while cooling decreases it

• Mechanical Property Variation with Temperature:

  • Indicates suitability for hard or soft tissue devices

• Reference: Schut et al. on glass transition temperature predictions

Thermal Transition

• Phase Transition at 0°C:

  • Molecules change phase when thermal energy is added

Thermal Transition in Biomaterials

• Deformation at Transition Points:

  • Importance of understanding deformation under operational conditions

  • Examples:

    • High-temperature applications for nanoparticles

    • Low-temperature uses such as stitches in colder climates

    • Collagen scaffolds changing state for easier injection

Transition Temperatures

• Crystalline Materials:

  • Melting point (Tm) signifies transition from solid to liquid phase

• Amorphous Ceramics:

  • Lacks distinct Tm, becomes increasingly viscous with decreasing temperature

Transition Temperatures: Tg and Tm

• Tg: Temperature where material changes from glassy to rubbery state

• Tm: Melting point where crystalline structure transitions to a liquid state

• Example: Glass vs. crystalline materials like metals

Characteristics of Polymers

• Behavior Variability:

  • Polymers may act as liquid, rubbery solid, or glass dependent on temperature and structure

• Crystalline vs. Amorphous:

  • Crystalline components exhibit distinct melting points (Tm)

  • Amorphous components transition via glass transition temperature (Tg)

Factors Influencing Tm in Polymers

• Molecular Weight:

  • Higher molecular weight correlates with increased Tm due to fewer chain ends to break

• Branching: Higher branching decreases Tm due to less dense packing and weaker interactions

Amorphous Polymers and Their Transition Points

• Characteristics:

  • Amorphous materials have glass transition temperature similar to amorphous ceramics

  • Tg generally lower than Tm

  • Example: PCL has Tm of 60°C and Tg of -60°C; Glass has higher Tg (140-370°C)

Chain Flexibility and Tg

• Influential Factors:

  • Flexibility affects Tg; more flexible chains lower Tg

  • Chemical constituents affect rotational freedom: C–O bonds more flexible than C–C.

  • Bulky side groups increase Tg due to limited backbone rotation

Semicrystalline Polymers

• Contain both crystalline and amorphous regions leading to:

  • Distinct Tg and Tm

  • Tm may be hard to detect in low crystalline polymers

Crystallization Temperature (Tc)

• Defined as the temperature for polymers with low crystallinity allowing orderly arrangement into crystalline state

• Exothermic Process: Arranging polymer chains into a crystalline state involves heat release

Problem: Ordering Tg

• Ranking and Rationale:

  • Materials should be placed according to chain flexibility and bulky groups' influence on Tg.

  • Example: Chain flexibility leads to lower Tg; bulky structures raise Tg.

Characterization Methods

• Thermal Analysis: Provides insights into material properties as a function of temperature

Differential Scanning Calorimetry (DSC)

• Assesses heat flow differences between sample and reference materials during temperature changes

• Two types: Power-compensated and Heat-flux DSC

Power-compensated DSC

• Uses separate heaters for sample/reference, maintaining near-zero temperature difference

Heat-flux DSC

• Heats both sample and reference from a common heater, measuring temperature difference as heat flow

Information Provided by DSC

• Useful for determining percent crystallinity in polymers through comparisons of Tm areas in different states

Problem Qualifying Transition Temperatures

• Matching temperature transitions (Tg, Tm, Tc) with respective thermogram points

Polymer Tragedy: Challenger Disaster

• In 1986, the Challenger shuttle disintegrated due to O-ring failure at low temperatures affecting mechanical properties

• Consequence: Stiffening of gasket compromised sealing, causing structural failure